#include <stdio.h>
#include <string.h>
#include <stdlib.h>

void replace(char oldword[], char newword[], char sentence[])
{
    int  j;

    int k    = 0;
    int len_old = strlen(oldword);
    int len_new = strlen(newword);
    int lenS    = strlen(sentence);

    int  maximum_result = (lenS / len_old + 1) * len_new + 1;   // think !?
    char result[maximum_result];

    for(int i=0 ; i<lenS ; i++) {
        for(j=0 ; j<len_old ; j++) {
            if(oldword[j] != sentence[j+i]) {
                break;
            }
        }
        if(j == len_old) {              // oldword is a substring of sentence
            for(j=0 ; j<len_new ; j++) {
                result[k++] = newword[j];
            }
            i += len_old-1;
        }
        else {                          // oldword not a substring of sentence
            result[k++] = sentence[i];
        }
    }
    result[k] = '\0';
    printf("%s\n",result);
}

int main(int argc, char *argv[])
{
    if(argc != 4) {
        fprintf(stderr,"Usage : %s str1 str2 str3\n",argv[0]);
        exit(1);
    }
    replace(argv[1],argv[2],argv[3]);
    return 0;
}