Which way should I follow to create an algorithm to find out whether fibonacci sequence exists in a given string ?
The string includes only digits with no whitespaces and there may be more than one sequence, I need to find all of them.
If as your comment says the first number must have less than 6 digits, you can simply search for all positions there one of the 25 fibonacci numbers (there are only 25 with less than 6 digits) and than try to expand this 1 number sequence in both directions.
After your update:
You can even speed things up when you are only looking for sequences of at least 3 numbers.
Prebuild all 25 3-number-Strings that start with one of the 25 first fibonnaci-numbers this should give much less matches than the search for the single fibonacci-numbers I suggested above.
Than search for them (like described above and try to expand the found 3-number-sequences).
here's how I would approach this.
The main algorithm could search for triplets then try to extend them to as long a sequence as possible.
This leaves us with the subproblem of finding triplets. So if you are scanning through a string to look for fibonacci numbers, one thing you can take advantage of is that the next number must have the same number of digits or one more digit.
e.g. if you have the string "987159725844" and are considering "[987]159725844" then the next thing you need to look at is "987[159]725844" and "987[1597]25844". Then the next part you would find is "[2584]4" or "[25844]".
Once you have the 3 numbers you can check if they form an arithmetic progression with C - B == B - A. If they do you can now check if they are from the fibonacci sequence by seeing if the ratio is roughly 1.6 and then running the fibonacci iteration backwards down to the initial conditions 1,1.
The overall algorithm would then work by scanning through looking for all triples starting with width 1, then width 2, width 3 up to 6.
I'd say you should first find all interesting Fibonacci items (which, having 6 or less digits, are no more than 30) and store them into an array.
Then, loop every position in your input string, and try to find upon there the longest possible Fibonacci number (that is, you must browse the array backwards).
If some Fib number is found, then you must bifurcate to a secondary algorithm, consisting of merely going through the array from current position to the end, trying to match every item in the following substring. When the matching ends, you must get back to the main algorithm to keep searching in the input string from the current position.
None of these two algorithms is recursive, nor too expensive.
update
Ok. If no tables are allowed, you could still use this approach replacing in the first loop the way to get the bext Fibo number: Instead of indexing, apply your formula.
Related
Given a string, we need to find the largest square which can be obtained by replace its characters by digits (leading zeros are not allowed) where same characters always map to the same digits and different characters always map to different digits. If no solution, return -1.
Consider the string "ab" If we replace character a with 8 and b with 1, we get 81, which is a square.
How to find it for given string ? It is given that string length can be at max 11.
Please help me find a suitable and efficient way
Sorry can't comment, not enough reputation for it so I'll answer here.
#mat7 about what you said in your question comments, no you don't have to do it for every letter from a to z. You only have to do it for the letters present in your string (so at max 12 letters, not 26).
The first thing I would even check is how much different letter you have, if it's 11 or 12 different letters you can directly return -1 since you can't have different letters having the same number.
Now, supposing the input string being "fdsadrtas", you take a new array with only each different letter => "fdsadrt"
And with this array you try all possibilities (exclude the obvious mismatching options, if you set 'f' to 4 and 'd' to 5, 's' can only be 12367890 (and f can never be 0)), this way you will exclude lots of possibilities, having as worst case 10! instead of 12^10. (actually 9*9! with the test of the first one never beeing 0 but it's close enough)
EDIT 2 : +1 samgak nice idea !
The last digit can only be 0,1,4,5,6,9 so the worst number of tests drop even to 9*6*8!
10! is by far small enough to be brute tested, keep the higher square value you found and you are done.
EDIT :
Actually It would work (in a finite reasonable amount of time) but it is the wrong approach now that I have thought about it.
You will use less time in looking all the squares numbers that could be a solution for your string, using the exemple I gave above it's a string of length 9, and checking each square who is length 9 if he could be successfully mapped into the string.
For a string of length 12 (the worst case) you will have to check the square values of 316'228 to 999'999, who is way less than the >2 millions check of the previous proposition. The other proposition might become faster if you start accepting long strings but with only 12 you are faster this way.
I'am looking for an algorithm with which it is possible to derive a key from an already happened shuffling-process.
Assume we've got the string "Hello" which was shuffled:
"hello" -> "loelh"
Now I would like to derive a key k from it which i could use to undo the shuffling. So if we use k as input parameter for a deterministic shuffling-algorithm like for example Fisher-Yates and shuffle "loelh" again, we would restore the initial string "hello".
What i do not mean is to simply use one and the same deterministic shuffling algorithm to shuffle and de-shuffle. That's because in my case the first string would not have been really shuffled in the classical sense. Actually there would be two sets of data (byte or bit-arrays) which are just given and we want to get from the first to the second one with just a key which has been derived before.
I hope it's clear what I want to achieve and I would appreciate all hints or proposed solutions.
Regards,
Merrit
UPDATE:
Another attemp:
basically, one could also call it deterministic transformation of a bunch of data e.g. a byte-array, but I will stick with the "hello"-string example.
Assume we've got a transformation-algorithm transform(data, "unknown seed") where data is "hello" and unknown seed is what we are looking for. The result of transform is "loelh". We are looking for this "unknown seed" which we could use to reverse the process. At the time of the "unknown seed"-generation, both, the input data AND the result are known of course.
Later on I want to use the "unknown seed" (which should be known already ;-) to get the original string again: so this transform("loelh", seed) should lead to "hello" again.
So you could also see it as a form of equation like data*["unknown value"]=resultdata and we are trying to find the unknown value (the operator * could be any kind of operation).
First of all, let's simplify the problem greatly. Instead of permuting "hello", let's assume that you are always permuting "abcde", as that will make it easier to understand.
A shuffle is the random generation of a permutation. How the shuffle generates the permutation is irrelevant; shuffles generate permutations, that's all we need to know.
Let's state a permutation as a string containing the numbers 1 through 5. Suppose the shuffle produces permutation "21453". That is, we take the first letter and put it in position 2: _a___. We take the second letter and put it in position 1, ba___. We take the 3rd letter and put it in position 5: ab__c. We take the fourth letter and put it in position 3, bad_c, and we take the fifth letter and put it in position 4, badec.
Now you wish to deduce a "key" which allows you to "unpermute" the permutation. Well, that's just another permutation, called the inverse permutation. To compute the inverse permutation of "21453" you do the following:
find "1". It's in the 2nd spot.
find "2". It's in the 1st spot.
find "3". It's in the 5th spot.
find "4". It's in the 3rd spot.
Find "5". It's in the 4th spot.
And now read down the second column; the inverse permutation of "21453" is "21534". We are unpermuting "badec". We put the first letter in position 2: _b___. We put the second letter in position 1: ab___. We put the third letter in position 4: ab_d_. We put the fourth letter in position 5: ab_de. And we put the fifth letter in position 3: abcde.
Shuffling is just creating a random permutation of a given sequence. The typical way to do that is something like the Fisher-Yates Shuffle that you pointed out. The problem is that the shuffle program generates multiple random numbers based on a seed, and unless you implement the random number generator there's no easy way to reverse the sequence of random numbers.
There is another way to do it. What if you could generate the nth permutation of a sequence directly? That is, given the string "Fast", you define the first few permutations as:
0 Fast
1 Fats
2 Fsat
3 Fsta
... etc. for all 24 permutations
You want a random permutation of those four characters. Select a random number from 0 to 23 and then call a function to generate that permutation.
If you know the key, you can call a different function, again passing that key, to have it reverse the permutation back to the original.
In the fourth article in his series on permutations, Eric Lippert showed how to generate the nth permutation without having to generate all of the permutations that come before it. He doesn't show how to reverse the process, but doing so shouldn't be difficult if you understand how the generator works. It's well worth the time to study the entire series of articles.
If you don't know what the key (i.e. the random number used) is, then deriving the sequence of swaps required to get to the original order is expensive.
Edit
Upon reflection, it just might be possible to derive the key if you're given the original sequence and the transformed sequence. Since you know how far each symbol has moved, you should be able to derive the key. Consider the possible permutations of two letters:
0. ab 1. ba
Now, assign the letter b the value of 0, and the letter a the value of 1. What permutation number is ba? Find a in the string, swap to the left until it gets to the proper position, and multiply the number of swaps by one.
That's too easy. Consider the next one:
0. abc 1. acb 2. bac
3. cab 4. bca 5. cba
a is now 2, b is 1, and c is 0. Given cab:
swap a left one space. 1x2 = 2. Result is `acb`
swap b left one space. 1x1 = 1. Result is `abc`
So cab is permutation #3.
This does assume that your permutation generator numbers the permutations in the same way. It's also not a terribly efficient way of doing things. Worst case will require n(n-1)/2 swaps. You can optimize the swaps by moving things in an array, but it's still an O(n^2) algorithm. Where n is the length of the sequence. Not terrible for 100 or maybe even 1,000 items. Pretty bad after that, though.
I am solving the 8-puzzle. It is a problem which looks like this:
Image courtesy of: https://ece.uwaterloo.ca/~dwharder/aads/Algorithms/N_puzzles/ (you can also see there for a more detailed description of the 8-puzzle). The user can move a square adjacent to the blank into the blank. The task is to restore the arrangement as shown in the picture, starting from some arbitrary arrangement.
Now, of course the state can be described as a permutation of 9 digits. In case of the picture shown, the permutation is:
1 2 3 4 5 6 7 8 0
However, not all permutations are reachable from the shown configuration. Therefore, I have the following questions.
What is the number of permutations obtainable from the shown initial configuration by sliding tiles into the blank?
Call the answer to the above N. Now, I want a 1-1 mapping from integers from 1 to N to permutations. That is, I want to have a function that takes a permutation and returns an appropriate integer as well as a function that takes an integer and returns the permutation. The mapping has to be a bijection (i.e. an imperfect hash is not enough).
181440.
Stick them in an array and sort it, e.g. lexicographically. Then converting integers to permutations is O(1), and going the other way is O(log n).
Well if you just want to enumerate the different possible states that can be reached, you can just depth first search from your initial state. It's very possible to generate the valid next states given a current state, for example: moving a tile down into the empty space is the same as swapping the 0 tile with the tile 3 before it in the permutation if there is one. So you just do a dfs and keep a hashset of all the permutations as your visited array which could be stored as ints or strings. there are only 9! possible states which is only 362880. If you need a 1-1 mapping from the set of integers just make the hashset a hashtable and everytime you find a new state just add it to the hash table at the next index. You could also find the shortest solution by doing a breadth first first instead and just breaking when you find the solved state.
I need to find if any permutation of the number exists within a specified range, i just need to return Yes or No.
For eg : Number = 122, and Range = [200, 250]. The answer would be Yes, as 221 exists within the range.
PS:
For the problem that i have in hand, the number to be searched
will only have two different digits (It will only contain 1 and 2,
Eg : 1112221121).
This is not a homework question. It was asked in an interview.
The approach I suggested was to find all permutations of the given number and check. Or loop through the range and check if we find any permutation of the number.
Checking every permutation is too expensive and unnecessary.
First, you need to look at them as strings, not numbers,
Consider each digit position as a seperate variable.
Consider how the set of possible digits each variable can hold is restricted by the range. Each digit/variable pair will be either (a) always valid (b) always invalid; or (c) its validity is conditionally dependent on specific other variables.
Now model these dependencies and independencies as a graph. As case (c) is rare, it will be easy to search in time proportional to O(10N) = O(N)
Numbers have a great property which I think can help you here:
For a given number a of value KXXXX, where K is given, we can
deduce that K0000 <= a < K9999.
Using this property, we can try to build a permutation which is within the range:
Let's take your example:
Range = [200, 250]
Number = 122
First, we can define that the first number must be 2. We have two 2's so we are good so far.
The second number must be be between 0 and 5. We have two candidate, 1 and 2. Still not bad.
Let's check the first value 1:
Any number would be good here, and we still have an unused 2. We have found our permutation (212) and therefor the answer is Yes.
If we did find a contradiction with the value 1, we need to backtrack and try the value 2 and so on.
If none of the solutions are valid, return No.
This Algorithm can be implemented using backtracking and should be very efficient since you only have 2 values to test on each position.
The complexity of this algorithm is 2^l where l is the number of elements.
You could try to implement some kind of binary search:
If you have 6 ones and 4 twos in your number, then first you have the interval
[1111112222; 2222111111]
If your range does not overlap with this interval, you are finished. Now split this interval in the middle, you get
(1111112222 + 222211111) / 2
Now find the largest number consisting of 1's and 2's of the respective number that is smaller than the split point. (Probably this step could be improved by calculating the split directly in some efficient way based on the 1 and 2 or by interpreting 1 and 2 as 0 and 1 of a binary number. One could also consider taking the geometric mean of the two numbers, as the candidates might then be more evenly distributed between left and right.)
[Edit: I think I've got it: Suppose the bounds have the form pq and pr (i.e. p is a common prefix), then build from q and r a symmetric string s with the 1's at the beginning and the end of the string and the 2's in the middle and take ps as the split point (so from 1111112222 and 1122221111 you would build 111122222211, prefix is p=11).]
If this number is contained in the range, you are finished.
If not, look whether the range is above or below and repeat with [old lower bound;split] or [split;old upper bound].
Suppose the range given to you is: ABC and DEF (each character is a digit).
Algorithm permutationExists(range_start, range_end, range_index, nos1, nos2)
if (nos1>0 AND range_start[range_index] < 1 < range_end[range_index] and
permutationExists(range_start, range_end, range_index+1, nos1-1, nos2))
return true
elif (nos2>0 AND range_start[range_index] < 2 < range_end[range_index] and
permutationExists(range_start, range_end, range_index+1, nos1, nos2-1))
return true
else
return false
I am assuming every single number to be a series of digits. The given number is represented as {numberOf1s, numberOf2s}. I am trying to fit the digits (first 1s and then 2s) within the range, if not the procudure returns a false.
PS: I might be really wrong. I dont know if this sort of thing can work. I haven't given it much thought, really..
UPDATE
I am wrong in the way I express the algorithm. There are a few changes that need to be done in it. Here is a working code (It worked for most of my test cases): http://ideone.com/1aOa4
You really only need to check at most TWO of the possible permutations.
Suppose your input number contains only the digits X and Y, with X<Y. In your example, X=1 and Y=2. I'll ignore all the special cases where you've run out of one digit or the other.
Phase 1: Handle the common prefix.
Let A be the first digit in the lower bound of the range, and let B be the first digit in the upper bound of the range. If A<B, then we are done with Phase 1 and move on to Phase 2.
Otherwise, A=B. If X=A=B, then use X as the first digit of the permutation and repeat Phase 1 on the next digit. If Y=A=B, then use Y as the first digit of the permutation and repeat Phase 1 on the next digit.
If neither X nor Y is equal to A and B, then stop. The answer is No.
Phase 2: Done with the common prefix.
At this point, A<B. If A<X<B, then use X as the first digit of the permutation and fill in the remaining digits however you want. The answer is Yes. (And similarly if A<Y<B.)
Otherwise, check the following four cases. At most two of the cases will require real work.
If A=X, then try using X as the first digit of the permutation, followed by all the Y's, followed by the rest of the X's. In other words, make the rest of the permutation as large as possible. If this permutation is in range, then the answer is Yes. If this permutation is not in range, then no permutation starting with X can succeed.
If B=X, then try using X as the first digit of the permutation, followed by the rest of the X's, followed by all the Y's. In other words, make the rest of the permutation as small as possible. If this permutation is in range, then the answer is Yes. If this permutation is not in range, then no permutation starting with X can succeed.
Similar cases if A=Y or B=Y.
If none of these four cases succeed, then the answer is No. Notice that at most one of the X cases and at most one of the Y cases can match.
In this solution, I've assumed that the input number and the two numbers in the range all contain the same number of digits. With a little extra work, the approach can be extended to cases where the numbers of digits differ.
Suppose there is given two String:
String s1= "MARTHA"
String s2= "MARHTA"
here we exchange positions of T and H. I am interested to write code which counts how many changes are necessary to transform from one String to another String.
There are several edit distance algorithms, the given Wikipeida link has links to a few.
Assuming that the distance counts only swaps, here is an idea based on permutations, that runs in linear time.
The first step of the algorithm is ensuring that the two strings are really equivalent in their character contents. This can be done in linear time using a hash table (or a fixed array that covers all the alphabet). If they are not, then s2 can't be considered a permutation of s1, and the "swap count" is irrelevant.
The second step counts the minimum number of swaps required to transform s2 to s1. This can be done by inspecting the permutation p that corresponds to the transformation from s1 to s2. For example, if s1="abcde" and s2="badce", then p=(2,1,4,3,5), meaning that position 1 contains element #2, position 2 contains element #1, etc. This permutation can be broke up into permutation cycles in linear time. The cycles in the example are (2,1) (4,3) and (5). The minimum swap count is the total count of the swaps required per cycle. A cycle of length k requires k-1 swaps in order to "fix it". Therefore, The number of swaps is N-C, where N is the string length and C is the number of cycles. In our example, the result is 2 (swap 1,2 and then 3,4).
Now, there are two problems here, and I think I'm too tired to solve them right now :)
1) My solution assumes that no character is repeated, which is not always the case. Some adjustment is needed to calculate the swap count correctly.
2) My formula #MinSwaps=N-C needs a proof... I didn't find it in the web.
Your problem is not so easy, since before counting the swaps you need to ensure that every swap reduces the "distance" (in equality) between these two strings. Then actually you look for the count but you should look for the smallest count (or at least I suppose), otherwise there exists infinite ways to swap a string to obtain another one.
You should first check which charaters are already in place, then for every character that is not look if there is a couple that can be swapped so that the next distance between strings is reduced. Then iterate over until you finish the process.
If you don't want to effectively do it but just count the number of swaps use a bit array in which you have 1 for every well-placed character and 0 otherwise. You will finish when every bit is 1.