样例过了 却只有30 求神犇帮助

#include <bits/stdc++.h>
#define init ios::sync_with_stdio(false), cin.tie(0), cout.tie(0)
using namespace std;

typedef long long ll;

ll n, q[1000001], q1[1000001], a[1001][1001], h, t, ans[1000001];
ll x[2], y[2];
ll dx[4] = {1, 0, 0, -1}, dy[4] = {0, 1, -1, 0};

int main() {
    init;
    cin >> n;
    for (ll i = 1; i <= n; i++)  {
        for (ll j = 1; j <= n; j++) {
        	char c;
            cin >> c;
            a[i][j] = c - '0';
        }
    }
    cin >> x[0] >> y[0] >> x[1] >> y[1];
    t++;
    q[t] = x[0];
    q1[t] = y[0];
    a[x[0]][y[0]] = 1;
    while (h < t) {
        h++;
        for (ll i = 0; i < 4; i++) {
            ll xa = q[h]+dx[i], ya = q1[h]+dy[i];
            if (xa>0 && ya>0 && xa<=n && ya<=n && !a[xa][ya]) {
                t++;
                q[t] = xa;
                q1[t] = ya;
                a[xa][ya] = 1;
                ans[t] = ans[h]+1;
                if (q[t] == x[1] && q1[t] == y[1]) {
                    cout << ans[t];
                    return 0;
                }
            }
        }
    }
    return 0;
}