c++35分求助

#include<bits/stdc++.h>
using namespace std;
int a[505][505];//记录每个点流星砸下的时间
int t[505][505];//记录奶牛走到每个点所需的时间
struct node { int x, y; };
queue<node> qu;
int dx[5] = { -1,0,0,1 };
int dy[5] = { 0,-1,1,0 };
int M;
int now;
int main() {
    cin >> M;
    memset(a, -1, sizeof(a));
    for (int i = 1; i <= M; i++)
    {
        int x, y, time;
        cin >> x >> y >> time;
        if (a[x][y] == -1 || time <= a[x][y])
        {
            a[x][y] = time;
            for (int j = 0; j < 4; j++)
            {
                int nx = x + dx[j], ny = y + dy[j];
                if (nx >= 0 && ny >= 0 && (time < a[nx][ny] || a[nx][ny] == -1))a[nx][ny] = time;
            }
        }
    }
    qu.push({ 0,0 });
    t[0][0] = 0;
    if (a[0][0] == -1)
    {
        cout << 0;
        return 0;
    }
    while (!qu.empty())
    {
        int x0 = qu.front().x, y0 = qu.front().y;
        qu.pop();
        for (int i = 0; i < 4; i++)
        {
            int nx = x0 + dx[i], ny = y0 + dy[i];
            if (nx >= 0 && ny >= 0 && (a[nx][ny] > t[x0][y0] + 1 || a[nx][ny] == -1))
            {
                t[nx][ny] = t[x0][y0] + 1;
                if (a[nx][ny] == -1)
                {
                    cout << t[nx][ny];
                    return 0;
                }
                qu.push({ nx,ny });
            }
        }
    }
    cout << -1;
    return 0;
}