Codeforces Round #387 (Div. 2), problem: (B) Mammoth’s Genome Decoding Solution in C/C++

Codeforces Round #387 (Div. 2), problem: (B) Mammoth’s Genome Decoding Solution in C/C++

#include <stdio.h>

char cc[] = { 'A', 'C', 'G', 'T' };
int cnt[4];

void incr(char c) {
	int i;

	for (i = 0; i < 4; i++)
		if (cc[i] == c) {
			cnt[i]++;
			return;
		}
}

int main() {
	int i, j, n, yes;
	static char s[512];

	scanf("%d", &n);
	scanf("%s", s);
	yes = 1;
	if (n % 4 != 0)
		yes = 0;
	for (i = 0; i < n; i++)
		if (s[i] != '?')
			incr(s[i]);
	for (i = 0; i < n; i++) {
		if (cnt[i] > n / 4) {
			yes = 0;
			break;
		}
		cnt[i] = n / 4 - cnt[i];
	}
	for (i = j = 0; i < n; i++) {
		while (cnt[j] == 0)
			j++;
		if (s[i] == '?') {
			cnt[j]--;
			s[i] = cc[j];
		}
	}
	printf("%s\n", yes ? s : "===");
	return 0;
}