40pts，AC on 答案为 No 的点，必关

#include<bits/stdc++.h>
using namespace std;
bool a[105][105];
bool flag = 0;
int n, m;
void dfs(int x, int y){
    if(!a[x][y] || x < 1 || y < 1 || x > n || y > m){
        return;
    }
    a[x][y] = 0;
    if(x == 1 && y == 1){
        flag = 1;
        return;
    }
    dfs(x, y - 1);
    dfs(x - 1, y);
    dfs(x, y + 1);
    dfs(x + 1, y);
}
int main(){
    cin >> n >> m;
    char c;
    for(int i = 1; i <= n; i++){
        for(int j = 1; j <= m; j++){
            cin >> c;
            if(c == '.') a[i][j] = true;
            else a[i][j] = false;
        }
    }
    cout << (flag ? "Yes" : "No");
}