P1363-幻象迷宫

#include <bits/stdc++.h>
#define _for(i,a,b) for(int i = (a);i < b;i ++)
typedef long long ll;
using namespace std;
struct point
{
    int x;
    int y;
    bool operator == (point b)
    {
        return this->x==b.x && this->y==b.y;
    }
};
typedef pair<point,point> pir;

inline ll read()
{
    ll ans = 0;
    char ch = getchar(), last = ' ';
    while(!isdigit(ch)) last = ch, ch = getchar();
    while(isdigit(ch)) ans = (ans << 1) + (ans << 3) + ch - '0', ch = getchar();
    if(last == '-') ans = -ans;
    return ans;
}
inline void write(ll x)
{
    if(x < 0) x = -x, putchar('-');
    if(x >= 10) write(x / 10);
    putchar(x % 10 + '0');
}

int N,M;
char m[1504][1504];
bool vis[1504][1504];
point pre[1504][1504];

point st;
int dx[] = {1,-1,0,0};
int dy[] = {0,0,1,-1};
bool bfs()
{
    queue<pir> q;
    q.push({st,st});
    vis[st.x][st.y] = 1;
    pre[st.x][st.y] = st;
    while(!q.empty())
    {
        pir cur = q.front();q.pop();
        point a = cur.first;
        point b = cur.second;

        _for(i,0,4)
        {
            point ta;
            ta.x = (a.x+dx[i]+N)%N;
            ta.y = (a.y+dy[i]+M)%M;
            point tb;
            tb.x = b.x+dx[i];
            tb.y = b.y+dy[i];

            if(vis[ta.x][ta.y] && !(tb==pre[ta.x][ta.y]))
                return true;

            if(vis[ta.x][ta.y] || m[ta.x][ta.y]=='#')
                continue;

            vis[ta.x][ta.y] = 1,pre[ta.x][ta.y] = tb;
            q.push({ta,tb});
        }
    }
    return false;
}
int main()
{
    while(scanf("%d%d",&N,&M)!=EOF)
    {
        memset(m,0,sizeof(m));
        memset(vis,0,sizeof(vis));
        memset(pre,0,sizeof(pre));
        _for(i,0,N)
        _for(j,0,M)
        {
            cin >> m[i][j];
            if(m[i][j]=='S')
                st.x = i,st.y = j,m[i][j] = '.';
        }

        if(bfs())
            printf("Yes
");
        else
            printf("No
");
    }
    return 0;
}

https://www.luogu.org/blog/Asurudo/solution-p1363