Group all occurrences of characters according to first appearance: C Program

Given a string of lowercase characters, the task is to print the string in a manner such that a character comes first in string displays first with all its occurrences in string.
Examples:

Input :  str = "occurrence"
output : occcurreen 

Input  : str = "cdab"
Output : cdab

C Program

#include <stdio.h>

#include <string.h>

main()

{

char st[100],temp,s[100];

int i,k=0,j,flag=0;

gets(st);

               

for(i=0;st[i]!='\0';i++)

{

                for(j=i;st[j]!='\0';j++)

                {

                                if(k<strlen(st))

                                {

                                                if(st[i] == st[j])

                                                                s[k++]=st[j];      

                                }

                                else

                                {

                                                flag=1;

                                                break;

                                }                                             

                }

                if(flag ==1)

                                break;

}

s[k]='\0';

puts(s);

}