50142. Word Merge Sort, Again

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

int k;
char file_names[20][20];
char out_name[40];
FILE* f_in[20];
FILE* f_out;

int using_cnt;
int using[20];
char list[20][3];

int read_chars(int idx){
    int rtv = fscanf(f_in[idx], "%c%c%c", &list[idx][0], &list[idx][1], &list[idx][2]);
    return rtv;
}
void write_chars(FILE* f_out, int idx){
    fprintf(f_out, "%c%c%c", list[idx][0], list[idx][1], list[idx][2]);
}
// void print_chars(int idx){
//     printf("%c%c%c", list[idx][0], list[idx][1], list[idx][2]);
// }

int hash(int a){
    return (list[a][0] - 'a') * (26 * 26) +
           (list[a][1] - 'a') * (26) +
           (list[a][2] - 'a');
}

void solve(){
    while(using_cnt > 0){
        int min_idx = -1, min_val = INT_MAX;
        for(int i = 0; i < k; i++){
            if(using[i] == 0) continue;
            int h = hash(i);
            if(h < min_val) min_idx = i, min_val = h;
        }
        if(min_idx == -1) break;
        write_chars(f_out, min_idx);
        int rtv = read_chars(min_idx);
        if(rtv == EOF) using[min_idx] = 0, using_cnt--;
    }
}

int main(){
    scanf("%d", &k);
    for(int i = 0; i < k; i++) scanf("%s", file_names[i]);
    scanf("%s", out_name);

    for(int i = 0; i < k; i++) f_in[i] = fopen(file_names[i], "r"), using[i] = 1;
    f_out = fopen(out_name, "w");

    using_cnt = k;
    for(int i = 0; i < k; i++){
        int rtv = read_chars(i);
        if(rtv == EOF) using[i] = 0, using_cnt--;
    }
    solve();

    for(int i = 0; i < k; i++) fclose(f_in[i]);
    fclose(f_out);
}