哪位大佬看看这个哪有问题为什么得不出答案谢谢啦

#include<iostream>
#include<queue>
using namespace std;
#include<cstring>
int n,a,b;
int dx[]={-1,0,1,0};
int dy[]={0,1,0,-1};
int map[305][305];
typedef pair<int,int>PII;
queue <PII> q;
int dist[305][305];
int bfs(int x,int y){
    memset(dist,-1,sizeof dist);
    q.push({x,y});
    dist[x][y]=0;
    for(int i=0;i<4;i++){
        auto t=q.front();
        q.pop();
        a=t.first+dx[i],b=t.second+dy[i];
        if(a<0 || b<0 )continue;
        if(dist[t.first][t.second]+1>=map[a][b])continue;
        dist[a][b]=dist[t.first][t.second]+1;
        q.push({a,b});
        if(map[a][b]==1e9)break;
    }
    return dist[a][b];
}
int main(){
    memset(map,1e9,sizeof map);
    cin>>n;
    while(n--){
        int x1,y1,t1;
        cin>>x1>>y1>>t1;
        map[x1][y1]=min(t1,map[x1][y1]);
        for(int i=0;i<4;i++){
            map[x1][y1]=t1;
            int c=x1+dx[i],d=y1+dy[i];
            if(c<0 || c>301 || d<0 || d>301){
                map[c][d]=min(t1,map[c][d]);
            }
        }
    }
    int res=bfs(0,0);
    cout<<res;
}