Efficient Way to Arrange Odd and Even Data Sequentially - performance

Question

I have data like (1,2,3,4,5,6,7,8) .I want to arrange them in a way like (1,3,5,7,2,4,6,8) in n/2-2 swap without using any array and loop must be use 1 or less.
Note that i have to do the swap in existing array of number.If there is other way like without swap and without extra array use,
Please give me some advice.

Answer 1

maintain two pointers: p1,p2. p1 goes from start to end, p2 goes from end to start, and swap non matching elements.
pseudo code:
specialSort(array):
p1 <- array.start()
p2 <- array.end()
while (p1 != p2):
if (*p1 %2 == 0):
p1 <- p1 + 1;
continue;
if (*p2 %2 == 1):
p2 <- p2 -1;
continue;
//when here, both p1 and p2 need a swap
swap(p1,p2);
Note that complexity is O(n), at least one of p1 or p2 changes in every second iteration, so the loop cannot repeat more the 2*n=O(n) times. [we can find better bound, but it is not needed]. space complexity is trivially O(1), we allocate a constant amount of space: 2 pointers only.
Note2: if your language does not support pointers [i.e. java,ml,...], it can be replaced with indexes: i1 going from start to end, i2 going from end to start, with the same algorithm principle.

Answer 2

#include <stdio.h>
#include <string.h>
char array[26] = "ABcdEfGiHjklMNOPqrsTUVWxyZ" ;
#define COUNTOF(a_) (sizeof(a_)/sizeof(a_)[0])
#define IS_ODD(e) ((e)&0x20)
#define IS_EVEN(e) (!IS_ODD(e))
void doswap (char *ptr, unsigned sizl, unsigned sizr);
int main(void)
{
unsigned bot,limit,cut,top,size;
size = COUNTOF(array);
printf("Before:%26.26s\n", array);
/* pass 1 count the number of EVEN chars */
for (limit=top=0; top < size; top++) {
if ( IS_EVEN( array[top] ) ) limit++;
}
/* skip initial segment of EVEN */
for (bot=0; bot < limit;bot++ ) {
if ( IS_ODD(array[bot])) break;
}
/* Find leading strech of misplaced ODD + trailing stretch of EVEN */
for (cut=bot;bot < limit; cut = top) {
/* count misplaced items */
for ( ;cut < size && IS_ODD(array[cut]); cut++) {;}
/* count shiftable items */
for (top=cut;top < size && IS_EVEN(array[top]); top++) {;}
/* Now, [bot...cut) and [cut...top) are two blocks
** that need to be swapped: swap them */
doswap(array+bot, cut-bot, top-cut);
bot += top-cut;
}
printf("Result:%26.26s\n", array);
return 0;
}
void doswap (char *ptr, unsigned sizl, unsigned sizr)
{
if (!sizl || !sizr) return;
if (sizl >= sizr) {
char tmp[sizr];
memcpy(tmp, ptr+sizl, sizr);
memmove(ptr+sizr, ptr, sizl);
memcpy(ptr, tmp, sizr);
}
else {
char tmp[sizr];
memcpy(tmp, ptr, sizl);
memmove(ptr, ptr+sizl, sizr);
memcpy(ptr+sizl, tmp, sizl);
}
}