50133. Word Merge Sort

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <ctype.h>
//    v
// OOOOOOOOO
 
int compare(char* a, char* b){
    if(*a != *b) return (*a) - (*b);
    a++, b++;
    if(*a != *b) return (*a) - (*b);
    a++, b++;
    return (*a) - (*b);
}
 
void merge(char* a, char* b, char* c){
    int n = strlen(a), m = strlen(b);
    int ptr_a = 0, ptr_b = 0, ptr_c = 0;
 
    while(ptr_a < n && ptr_b < m){
        int rtv = compare(a + ptr_a, b + ptr_b);
        if(rtv <= 0){
            memcpy(c + ptr_c, a + ptr_a, sizeof(char) * 3);
            ptr_a += 3;
        }
        else{
            memcpy(c + ptr_c, b + ptr_b, sizeof(char) * 3);
            ptr_b += 3;
        }
        ptr_c += 3;
    }
    while(ptr_a < n){
        memcpy(c + ptr_c, a + ptr_a, sizeof(char) * 3);
        ptr_a += 3, ptr_c += 3;
    }
    while(ptr_b < m){
        memcpy(c + ptr_c, b + ptr_b, sizeof(char) * 3);
        ptr_b += 3, ptr_c += 3;
    }
}
 
int main(){
    char a[100005], b[100005], c[100005];
    scanf("%s", a);
 
    int flag = 1;
    while(scanf("%s", b) != EOF){
        if(flag == 1){
            merge(a, b, c);
            printf("%s\n", c);
            flag = 0;
        }
        else{
            merge(c, b, a);
            printf("%s\n", a);
            flag = 1;
        }
    }
}