Find the first un-repeated character in a string - algorithm

Question

What is the quickest way to find the first character which only appears once in a string?

Answer 1

It has to be at least O(n) because you don't know if a character will be repeated until you've read all characters.
So you can iterate over the characters and append each character to a list the first time you see it, and separately keep a count of how many times you've seen it (in fact the only values that matter for the count is "0", "1" or "more than 1").
When you reach the end of the string you just have to find the first character in the list that has a count of exactly one.
Example code in Python:
def first_non_repeated_character(s):
counts = defaultdict(int)
l = []
for c in s:
counts[c] += 1
if counts[c] == 1:
l.append(c)
for c in l:
if counts[c] == 1:
return c
return None
This runs in O(n).

Answer 2

I see that people have posted some delightful answers below, so I'd like to offer something more in-depth.
An idiomatic solution in Ruby
We can find the first un-repeated character in a string like so:
def first_unrepeated_char string
string.each_char.tally.find { |_, n| n == 1 }.first
end
How does Ruby accomplish this?
Reading Ruby's source
Let's break down the solution and consider what algorithms Ruby uses for each step.
First we call each_char on the string. This creates an enumerator which allows us to visit the string one character at a time. This is complicated by the fact that Ruby handles Unicode characters, so each value we get from the enumerator can be a variable number of bytes. If we know our input is ASCII or similar, we could use each_byte instead.
The each_char method is implemented like so:
rb_str_each_char(VALUE str)
{
RETURN_SIZED_ENUMERATOR(str, 0, 0, rb_str_each_char_size);
return rb_str_enumerate_chars(str, 0);
}
In turn, rb_string_enumerate_chars is implemented as:
rb_str_enumerate_chars(VALUE str, VALUE ary)
{
VALUE orig = str;
long i, len, n;
const char *ptr;
rb_encoding *enc;
str = rb_str_new_frozen(str);
ptr = RSTRING_PTR(str);
len = RSTRING_LEN(str);
enc = rb_enc_get(str);
if (ENC_CODERANGE_CLEAN_P(ENC_CODERANGE(str))) {
for (i = 0; i < len; i += n) {
n = rb_enc_fast_mbclen(ptr + i, ptr + len, enc);
ENUM_ELEM(ary, rb_str_subseq(str, i, n));
}
}
else {
for (i = 0; i < len; i += n) {
n = rb_enc_mbclen(ptr + i, ptr + len, enc);
ENUM_ELEM(ary, rb_str_subseq(str, i, n));
}
}
RB_GC_GUARD(str);
if (ary)
return ary;
else
return orig;
}
From this we can see that it calls rb_enc_mbclen (or its fast version) to get the length (in bytes) of the next character in the string so that it can iterate the next step. By lazily iterating over a string, reading just one character at a time, we end up doing just one full pass over the input string as tally consumes the iterator.
Tally is then implemented like so:
static void
tally_up(VALUE hash, VALUE group)
{
VALUE tally = rb_hash_aref(hash, group);
if (NIL_P(tally)) {
tally = INT2FIX(1);
}
else if (FIXNUM_P(tally) && tally < INT2FIX(FIXNUM_MAX)) {
tally += INT2FIX(1) & ~FIXNUM_FLAG;
}
else {
tally = rb_big_plus(tally, INT2FIX(1));
}
rb_hash_aset(hash, group, tally);
}
static VALUE
tally_i(RB_BLOCK_CALL_FUNC_ARGLIST(i, hash))
{
ENUM_WANT_SVALUE();
tally_up(hash, i);
return Qnil;
}
Here, tally_i uses RB_BLOCK_CALL_FUNC_ARGLIST to call repeatedly to tally_up, which updates the tally hash on every iteration.
Rough time & memory analysis
The each_char method doesn't allocate an array to eagerly hold the characters of the string, so it has a small constant memory overhead. When we tally the characters, we allocate a hash and put our tally data into it which in the worst case scenario can take up as much memory as the input string times some constant factor.
Time-wise, tally does a full scan of the string, and calling find to locate the first non-repeated character will scan the hash again, each of which carry O(n) worst-case complexity.
However, tally also updates a hash on every iteration. Updating the hash on every character can be as slow as O(n) again, so the worst case complexity of this Ruby solution is perhaps O(n^2).
However, under reasonable assumptions, updating a hash has an O(1) complexity, so we can expect the average case amortized to look like O(n).
My old accepted answer in Python
You can't know that the character is un-repeated until you've processed the whole string, so my suggestion would be this:
def first_non_repeated_character(string):
chars = []
repeated = []
for character in string:
if character in chars:
chars.remove(character)
repeated.append(character)
else:
if not character in repeated:
chars.append(character)
if len(chars):
return chars[0]
else:
return False
Edit: originally posted code was bad, but this latest snippet is Certified To Work On Ryan's Computer™.

Answer 3

Why not use a heap based data structure such as a minimum priority queue. As you read each character from the string, add it to the queue with a priority based on the location in the string and the number of occurrences so far. You could modify the queue to add priorities on collision so that the priority of a character is the sum of the number appearances of that character. At the end of the loop, the first element in the queue will be the least frequent character in the string and if there are multiple characters with a count == 1, the first element was the first unique character added to the queue.

Answer 4

Here is another fun way to do it. Counter requires Python2.7 or Python3.1
>>> from collections import Counter
>>> def first_non_repeated_character(s):
... return min((k for k,v in Counter(s).items() if v<2), key=s.index)
...
>>> first_non_repeated_character("aaabbbcddd")
'c'
>>> first_non_repeated_character("aaaebbbcddd")
'e'

Answer 5

Lots of answers are attempting O(n) but are forgetting the actual costs of inserting and removing from the lists/associative arrays/sets they're using to track.
If you can assume that a char is a single byte, then you use a simple array indexed by the char and keep a count in it. This is truly O(n) because the array accesses are guaranteed O(1), and the final pass over the array to find the first element with 1 is constant time (because the array has a small, fixed size).
If you can't assume that a char is a single byte, then I would propose sorting the string and then doing a single pass checking adjacent values. This would be O(n log n) for the sort plus O(n) for the final pass. So it's effectively O(n log n), which is better than O(n^2). Also, it has virtually no space overhead, which is another problem with many of the answers that are attempting O(n).

Answer 6

Counter requires Python2.7 or Python3.1
>>> from collections import Counter
>>> def first_non_repeated_character(s):
... counts = Counter(s)
... for c in s:
... if counts[c]==1:
... return c
... return None
...
>>> first_non_repeated_character("aaabbbcddd")
'c'
>>> first_non_repeated_character("aaaebbbcddd")
'e'

Answer 7

Refactoring a solution proposed earlier (not having to use extra list/memory). This goes over the string twice. So this takes O(n) too like the original solution.
def first_non_repeated_character(s):
counts = defaultdict(int)
for c in s:
counts[c] += 1
for c in s:
if counts[c] == 1:
return c
return None

Answer 8

The following is a Ruby implementation of finding the first nonrepeated character of a string:
def first_non_repeated_character(string)
string1 = string.split('')
string2 = string.split('')
string1.each do |let1|
counter = 0
string2.each do |let2|
if let1 == let2
counter+=1
end
end
if counter == 1
return let1
break
end
end
end
p first_non_repeated_character('dont doddle in the forest')
And here is a JavaScript implementation of the same style function:
var first_non_repeated_character = function (string) {
var string1 = string.split('');
var string2 = string.split('');
var single_letters = [];
for (var i = 0; i < string1.length; i++) {
var count = 0;
for (var x = 0; x < string2.length; x++) {
if (string1[i] == string2[x]) {
count++
}
}
if (count == 1) {
return string1[i];
}
}
}
console.log(first_non_repeated_character('dont doddle in the forest'));
console.log(first_non_repeated_character('how are you today really?'));
In both cases I used a counter knowing that if the letter is not matched anywhere in the string, it will only occur in the string once so I just count it's occurrence.

Answer 9

I think this should do it in C. This operates in O(n) time with no ambiguity about order of insertion and deletion operators. This is a counting sort (simplest form of a bucket sort, which itself is the simple form of a radix sort).
unsigned char find_first_unique(unsigned char *string)
{
int chars[256];
int i=0;
memset(chars, 0, sizeof(chars));
while (string[i++])
{
chars[string[i]]++;
}
i = 0;
while (string[i++])
{
if (chars[string[i]] == 1) return string[i];
}
return 0;
}

Answer 10

In Ruby:
(Original Credit: Andrew A. Smith)
x = "a huge string in which some characters repeat"
def first_unique_character(s)
s.each_char.detect { |c| s.count(c) == 1 }
end
first_unique_character(x)
=> "u"

Answer 11

def first_non_repeated_character(string):
chars = []
repeated = []
for character in string:
if character in repeated:
... discard it.
else if character in chars:
chars.remove(character)
repeated.append(character)
else:
if not character in repeated:
chars.append(character)
if len(chars):
return chars[0]
else:
return False

Answer 12

Other JavaScript solutions are quite c-style solutions here is a more JavaScript-style solution.
var arr = string.split("");
var occurences = {};
var tmp;
var lowestindex = string.length+1;
arr.forEach( function(c){
tmp = c;
if( typeof occurences[tmp] == "undefined")
occurences[tmp] = tmp;
else
occurences[tmp] += tmp;
});
for(var p in occurences) {
if(occurences[p].length == 1)
lowestindex = Math.min(lowestindex, string.indexOf(p));
}
if(lowestindex > string.length)
return null;
return string[lowestindex];
}

Answer 13

in C, this is almost Shlemiel the Painter's Algorithm (not quite O(n!) but more than 0(n2)).
But will outperform "better" algorithms for reasonably sized strings because O is so small. This can also easily tell you the location of the first non-repeating string.
char FirstNonRepeatedChar(char * psz)
{
for (int ii = 0; psz[ii] != 0; ++ii)
{
for (int jj = ii+1; ; ++jj)
{
// if we hit the end of string, then we found a non-repeat character.
//
if (psz[jj] == 0)
return psz[ii]; // this character doesn't repeat
// if we found a repeat character, we can stop looking.
//
if (psz[ii] == psz[jj])
break;
}
}
return 0; // there were no non-repeating characters.
}
edit: this code is assuming you don't mean consecutive repeating characters.

Answer 14

Here's an implementation in Perl (version >=5.10) that doesn't care whether the repeated characters are consecutive or not:
use strict;
use warnings;
foreach my $word(#ARGV)
{
my #distinct_chars;
my %char_counts;
my #chars=split(//,$word);
foreach (#chars)
{
push #distinct_chars,$_ unless $_~~#distinct_chars;
$char_counts{$_}++;
}
my $first_non_repeated="";
foreach(#distinct_chars)
{
if($char_counts{$_}==1)
{
$first_non_repeated=$_;
last;
}
}
if(length($first_non_repeated))
{
print "For \"$word\", the first non-repeated character is '$first_non_repeated'.\n";
}
else
{
print "All characters in \"$word\" are repeated.\n";
}
}
Storing this code in a script (which I named non_repeated.pl) and running it on a few inputs produces:
jmaney> perl non_repeated.pl aabccd "a huge string in which some characters repeat" abcabc
For "aabccd", the first non-repeated character is 'b'.
For "a huge string in which some characters repeat", the first non-repeated character is 'u'.
All characters in "abcabc" are repeated.

Answer 15

Here's a possible solution in ruby without using Array#detect (as in this answer). Using Array#detect makes it too easy, I think.
ALPHABET = %w(a b c d e f g h i j k l m n o p q r s t u v w x y z)
def fnr(s)
unseen_chars = ALPHABET.dup
seen_once_chars = []
s.each_char do |c|
if unseen_chars.include?(c)
unseen_chars.delete(c)
seen_once_chars << c
elsif seen_once_chars.include?(c)
seen_once_chars.delete(c)
end
end
seen_once_chars.first
end
Seems to work for some simple examples:
fnr "abcdabcegghh"
# => "d"
fnr "abababababababaqababa"
=> "q"
Suggestions and corrections are very much appreciated!

Answer 16

Try this code:
public static String findFirstUnique(String str)
{
String unique = "";
foreach (char ch in str)
{
if (unique.Contains(ch)) unique=unique.Replace(ch.ToString(), "");
else unique += ch.ToString();
}
return unique[0].ToString();
}

Answer 17

In Mathematica one might write this:
string = "conservationist deliberately treasures analytical";
Cases[Gather # Characters # string, {_}, 1, 1][[1]]
{"v"}

Answer 18

This snippet code in JavaScript
var string = "tooth";
var hash = [];
for(var i=0; j=string.length, i<j; i++){
if(hash[string[i]] !== undefined){
hash[string[i]] = hash[string[i]] + 1;
}else{
hash[string[i]] = 1;
}
}
for(i=0; j=string.length, i<j; i++){
if(hash[string[i]] === 1){
console.info( string[i] );
return false;
}
}
// prints "h"

Answer 19

Different approach here.
scan each element in the string and create a count array which stores the repetition count of each element.
Next time again start from first element in the array and print the first occurrence of element with count = 1
C code
-----
#include <stdio.h>
#include <stdlib.h>
int main(int argc, char *argv[])
{
char t_c;
char *t_p = argv[1] ;
char count[128]={'\0'};
char ch;
for(t_c = *(argv[1]); t_c != '\0'; t_c = *(++t_p))
count[t_c]++;
t_p = argv[1];
for(t_c = *t_p; t_c != '\0'; t_c = *(++t_p))
{
if(count[t_c] == 1)
{
printf("Element is %c\n",t_c);
break;
}
}
return 0;
}

Answer 20

input is = aabbcddeef output is = c
char FindUniqueChar(char *a)
{
int i=0;
bool repeat=false;
while(a[i] != '\0')
{
if (a[i] == a[i+1])
{
repeat = true;
}
else
{
if(!repeat)
{
cout<<a[i];
return a[i];
}
repeat=false;
}
i++;
}
return a[i];
}

Answer 21

Here is another approach...we could have a array which will store the count and the index of the first occurrence of the character. After filling up the array we could jst traverse the array and find the MINIMUM index whose count is 1 then return str[index]
#include <iostream>
#include <cstdio>
#include <cstdlib>
#include <climits>
using namespace std;
#define No_of_chars 256
//store the count and the index where the char first appear
typedef struct countarray
{
int count;
int index;
}countarray;
//returns the count array
countarray *getcountarray(char *str)
{
countarray *count;
count=new countarray[No_of_chars];
for(int i=0;i<No_of_chars;i++)
{
count[i].count=0;
count[i].index=-1;
}
for(int i=0;*(str+i);i++)
{
(count[*(str+i)].count)++;
if(count[*(str+i)].count==1) //if count==1 then update the index
count[*(str+i)].index=i;
}
return count;
}
char firstnonrepeatingchar(char *str)
{
countarray *array;
array = getcountarray(str);
int result = INT_MAX;
for(int i=0;i<No_of_chars;i++)
{
if(array[i].count==1 && result > array[i].index)
result = array[i].index;
}
delete[] (array);
return (str[result]);
}
int main()
{
char str[] = "geeksforgeeks";
cout<<"First non repeating character is "<<firstnonrepeatingchar(str)<<endl;
return 0;
}

Answer 22

Function:
This c# function uses a HashTable (Dictionary) and have a performance O(2n) worstcase.
private static string FirstNoRepeatingCharacter(string aword)
{
Dictionary<string, int> dic = new Dictionary<string, int>();
for (int i = 0; i < aword.Length; i++)
{
if (!dic.ContainsKey(aword.Substring(i, 1)))
dic.Add(aword.Substring(i, 1), 1);
else
dic[aword.Substring(i, 1)]++;
}
foreach (var item in dic)
{
if (item.Value == 1) return item.Key;
}
return string.Empty;
}
Example:
string aword = "TEETER";
Console.WriteLine(FirstNoRepeatingCharacter(aword)); //print: R

Answer 23

I have two strings i.e. 'unique' and 'repeated'. Every character appearing for the first time, gets added to 'unique'. If it is repeated for the second time, it gets removed from 'unique' and added to 'repeated'. This way, we will always have a string of unique characters in 'unique'.
Complexity big O(n)
public void firstUniqueChar(String str){
String unique= "";
String repeated = "";
str = str.toLowerCase();
for(int i=0; i<str.length();i++){
char ch = str.charAt(i);
if(!(repeated.contains(str.subSequence(i, i+1))))
if(unique.contains(str.subSequence(i, i+1))){
unique = unique.replaceAll(Character.toString(ch), "");
repeated = repeated+ch;
}
else
unique = unique+ch;
}
System.out.println(unique.charAt(0));
}

Answer 24

The following code is in C# with complexity of n.
using System;
using System.Linq;
using System.Text;
namespace SomethingDigital
{
class FirstNonRepeatingChar
{
public static void Main()
{
String input = "geeksforgeeksandgeeksquizfor";
char[] str = input.ToCharArray();
bool[] b = new bool[256];
String unique1 = "";
String unique2 = "";
foreach (char ch in str)
{
if (!unique1.Contains(ch))
{
unique1 = unique1 + ch;
unique2 = unique2 + ch;
}
else
{
unique2 = unique2.Replace(ch.ToString(), "");
}
}
if (unique2 != "")
{
Console.WriteLine(unique2[0].ToString());
Console.ReadLine();
}
else
{
Console.WriteLine("No non repeated string");
Console.ReadLine();
}
}
}
}

Answer 25

The following solution is an elegant way to find the first unique character within a string using the new features which have been introduced as part as Java 8. This solution uses the approach of first creating a map to count the number of occurrences of each character. It then uses this map to find the first character which occurs only once. This runs in O(N) time.
import static java.util.stream.Collectors.counting;
import static java.util.stream.Collectors.groupingBy;
import java.util.Arrays;
import java.util.List;
import java.util.Map;
// Runs in O(N) time and uses lambdas and the stream API from Java 8
// Also, it is only three lines of code!
private static String findFirstUniqueCharacterPerformantWithLambda(String inputString) {
// convert the input string into a list of characters
final List<String> inputCharacters = Arrays.asList(inputString.split(""));
// first, construct a map to count the number of occurrences of each character
final Map<Object, Long> characterCounts = inputCharacters
.stream()
.collect(groupingBy(s -> s, counting()));
// then, find the first unique character by consulting the count map
return inputCharacters
.stream()
.filter(s -> characterCounts.get(s) == 1)
.findFirst()
.orElse(null);
}

Answer 26

Here is one more solution with o(n) time complexity.
public void findUnique(String string) {
ArrayList<Character> uniqueList = new ArrayList<>();
int[] chatArr = new int[128];
for (int i = 0; i < string.length(); i++) {
Character ch = string.charAt(i);
if (chatArr[ch] != -1) {
chatArr[ch] = -1;
uniqueList.add(ch);
} else {
uniqueList.remove(ch);
}
}
if (uniqueList.size() == 0) {
System.out.println("No unique character found!");
} else {
System.out.println("First unique character is :" + uniqueList.get(0));
}
}

Answer 27

I read through the answers, but did not see any like mine, I think this answer is very simple and fast, am I wrong?
def first_unique(s):
repeated = []
while s:
if s[0] not in s[1:] and s[0] not in repeated:
return s[0]
else:
repeated.append(s[0])
s = s[1:]
return None
test
(first_unique('abdcab') == 'd', first_unique('aabbccdad') == None, first_unique('') == None, first_unique('a') == 'a')

Answer 28

Question : First Unique Character of a String
This is the simplest solution.
public class Test4 {
public static void main(String[] args) {
String a = "GiniGinaProtijayi";
firstUniqCharindex(a);
}
public static void firstUniqCharindex(String a) {
int[] count = new int[256];
for (int i = 0; i < a.length(); i++) {
count[a.charAt(i)]++;
}
int index = -1;
for (int i = 0; i < a.length(); i++) {
if (count[a.charAt(i)] == 1) {
index = i;
break;
} // if
}
System.out.println(index);// output => 8
System.out.println(a.charAt(index)); //output => P
}// end1
}
IN Python :
def firstUniqChar(a):
count = [0] * 256
for i in a: count[ord(i)] += 1
element = ""
for items in a:
if(count[ord(items) ] == 1):
element = items ;
break
return element
a = "GiniGinaProtijayi";
print(firstUniqChar(a)) # output is P
Using Java 8 :
public class Test2 {
public static void main(String[] args) {
String a = "GiniGinaProtijayi";
Map<Character, Long> map = a.chars()
.mapToObj(
ch -> Character.valueOf((char) ch)
).collect(
Collectors.groupingBy(
Function.identity(),
LinkedHashMap::new,
Collectors.counting()));
System.out.println("MAP => " + map);
// {G=2, i=5, n=2, a=2, P=1, r=1, o=1, t=1, j=1, y=1}
Character chh = map
.entrySet()
.stream()
.filter(entry -> entry.getValue() == 1L)
.map(entry -> entry.getKey())
.findFirst()
.get();
System.out.println("First Non Repeating Character => " + chh);// P
}// main
}

Answer 29

how about using a suffix tree for this case... the first unrepeated character will be first character of longest suffix string with least depth in tree..

Answer 30

Create Two list -
unique list - having only unique character .. UL
non-unique list - having only repeated character -NUL
for(char c in str) {
if(nul.contains(c)){
//do nothing
}else if(ul.contains(c)){
ul.remove(c);
nul.add(c);
}else{
nul.add(c);
}