79. SMS Language

Code:
  #include<stdio.h>
#include<string.h>
void replace(char *);
int main(){
  char s1[100];
  gets (s1);
  replace(s1);
  printf("\n%s",s1);
  return 0;
}
void replace(char *s1)
{
  char r[4][10]={"yes","you","today","why"};
    char rep[4][5]={"s","u","2day","y"};
      char buff[500];
  int i;
  char *ch;
  for(i=0;i<4;i++)
    if((ch=strstr(s1,r[i]))){
      strncpy(buff,s1,ch-s1);
      buff[ch-s1]=0;
      sprintf(buff+(ch - s1),"%s%s",rep[i],ch+strlen(r[i]));
      s1[0]=0;
      strcpy(s1,buff);
      return replace (s1);
    }
}