转化成编号，如果两个编号并起来之前就已经加入公共节点了，那么说明围城圈了

C++ 代码

#include <iostream>
#include <cstring>
#include <algorithm>

using namespace std;

const int N = 40010;

int n, m;
int p[N];

int find(int x)
{
    if (p[x] != x) p[x] = find(p[x]);
    return p[x];
}

int get(int x, int y)
{
    return x * n + y;
}

int main()
{
    int res = 0;
    cin >> n >> m;
    for (int i = 1; i <= n * n; i ++ ) p[i] = i;
    for (int i = 1; i <= m; i ++ )
    {
        int x, y;
        char c;
        cin >> x >> y >> c;
        x -- , y -- ;
        int a = get(x, y);
        int b;
        if (c == 'D') b = get(x + 1, y);
        else b = get(x, y + 1);

        int pa = find(a), pb = find(b);
        if (pa == pb)
        {
            res = i;
            break;
        }
        p[pa] = pb;
    }

    if (res == 0) cout << "draw" << endl;
    else cout << res << endl;
}