CodeCraft-20 (Div. 2)D（BFS）

#define HAVE_STRUCT_TIMESPEC
#include<bits/stdc++.h>
#include<tuple>
using namespace std;
int x[1007][1007],y[1007][1007];
queue<pair<int,int> >q,dq;
char ans[1007][1007];
typedef struct node{
    int x,y;
    char to,from;
};
node tle[7];
int main(){
    ios::sync_with_stdio(false);
    cin.tie(NULL);
    cout.tie(NULL);
    tle[1]={0,1,'R','L'};
    tle[2]={1,0,'D','U'};
    tle[3]={0,-1,'L','R'};
    tle[4]={-1,0,'U','D'};
    int n;
    cin>>n;
    for(int i=1;i<=n;++i){
        for(int j=1;j<=n;++j){
            cin>>x[i][j]>>y[i][j];
            if(x[i][j]==-1)
                q.push({i,j});
            if(x[i][j]==i&&y[i][j]==j){
                ans[i][j]='X';
                dq.push({i,j});
            }
        }
    }
    while(!q.empty()){
        pair<int,int>now=q.front();
        q.pop();
        for(int i=1;i<=4;++i){
            int dx=now.first+tle[i].x;
            int dy=now.second+tle[i].y;
            if(dx>0&&dx<=n&&dy>0&&dy<=n&&x[dx][dy]==-1){//把相邻的-1全都联通，让它们在-1的联通块里无限循环
                ans[now.first][now.second]=tle[i].to;
                ans[dx][dy]=tle[i].from;
                break;
            }
        }
    }
    while(!dq.empty()){//以不用动的点为起点BFS
        pair<int,int>now=dq.front();
        dq.pop();
        for(int i=1;i<=4;++i){
            int dx=now.first+tle[i].x;
            int dy=now.second+tle[i].y;
            if(dx>0&&dx<=n&&dy>0&&dy<=n&&x[dx][dy]==x[now.first][now.second]&&y[dx][dy]==y[now.first][now.second]&&ans[dx][dy]==0){
                ans[dx][dy]=tle[i].from;//有相同终点的点，把它连到原本的起点
                dq.push({dx,dy});//加入队列，这个点已经能走到它的终点，他自己也可以成为一个中间过度点
            }
        }
    }
    for(int i=1;i<=n;++i)
        for(int j=1;j<=n;++j)
            if(ans[i][j]==0){
                cout<<"INVALID";
                return 0;
            }
    cout<<"VALID
";
    for(int i=1;i<=n;++i){
        for(int j=1;j<=n;++j)
            cout<<ans[i][j];
        cout<<"
";
    }
    return 0;
}