用的并查集，大家凑合看看，等大佬来完善

C++ 代码

const int MAXN = 50005;
int f[MAXN];
int fid(int x){
    if(f[x] != x){
        x = fid(f[x]);
    }
    return f[x];
}

void uni(int x, int y){
    x = fid(x);
    y = fid(y);
    if(x != y){
        f[x] = y;
    }
}

class Solution {
public:
    int minReorder(int n, vector<vector<int>>& conn) {
        for(int i = 0; i < n; i++){
            f[i] = i;
        }
        int ans = 0;
        for(int i = 0; i < conn.size(); i++){
            if(fid(conn[i][0]) == 0){
                ans++;
                swap(conn[i][0], conn[i][1]);
            }
            uni(conn[i][0], conn[i][1]);
        }
        return ans;
    }
};