超时！

数据

输入：
11111111111111111111 2
1 2
2 1
应该输出
1048576

不应该啊

#include<map>
#include<cstdio>
#include<string>
#include<iostream>
#include<algorithm>
using namespace std;
string n, q[1000005];int len, k, x[20], y[20];
map<string, int> m1;
void Bfs() {
	int ans = 1, l = 1, r = 1;
	q[1] = n; m1[n] = 1;
	while(l <= r) {
		for(int i = 1; i <= k; ++i) {
			for(int j = 0; j < len; ++j) {
				//cout << q[l][j] << " " << x[i] << " "<< y[i] << endl;
				if (q[l][j] - '0' == x[i]) {
					string n1 = q[l];
					n1[j] = y[i] + '0';
					if (!m1[n1]) {
						q[++r] = n1;
						m1[n1] = 1;
						++ans;
					}
				}
			}
		}
		++l;
	}
	printf("%d\n", ans);
}
int main() {
	cin >> n >> k;
	len = n.size();
	for(int i = 1; i <= k; ++i)
		scanf("%d%d", x + i, y + i);
	Bfs();
	return 0;
}