题目1：108. 冗余连接 (kamacoder.com)

#include<iostream>
#include<vector>

using namespace std;

int n;
vector<int> father(10001, 0);

void init() {
    for(int i = 1;i <= n;i++) father[i] = i;
}

int find(int u) {
    return u == father[u] ? u : father[u] = find(father[u]); 
}

bool isSame(int u, int v) {
    u = find(u);
    v = find(v);
    return u == v;
}

void join(int u, int v) {
    u = find(u);
    v = find(v);
    if(u == v) return;
    father[v] = u;
}

int main() {
    cin >> n;
    init();
    int s, t;
    for(int i = 0;i < n;i++) {
        cin >> s >> t;
        if(isSame(s, t)) {
            cout << s << " " << t << endl;
        }else {
            join(s, t);
        }
    }
    return 0;
}

题目2：109. 冗余连接II (kamacoder.com)

三种情况：入度为2的时候，有两种情况一种是删哪个都行，另一种是删掉之后可能出现环，这时候就要判断删除这个边，是否成环了

如果没有入度为2，就是成环，这个从前向后遍历，通过查并集删除删除连通的即可

#include<iostream>
#include<vector>

using namespace std;

int n;
vector<int> father(1001, 0);

void init() {
    for(int i = 1;i <= n;i++) {
        father[i] = i;
    }
}

int find(int u) {
    return u == father[u] ? u : father[u] = find(father[u]);
}

bool same(int u, int v) {
    u = find(u);
    v = find(v);
    return u == v;
}

void join(int u, int v) {
    u = find(u);
    v = find(v);
    if(u == v) return;
    father[v] = u;
}

bool isTree(vector<vector<int>>& edge, int intnode) {
    init();
    for(int i = 0;i < n;i++) {
        if(i == intnode) continue;
        // 这里判断去掉这个边是否成环，这个实在入度为2的情况下
        if(same(edge[i][0], edge[i][1])) return false;
        else join(edge[i][0], edge[i][1]);
    }
    return true;
}

void getremovedge(vector<vector<int>>& edge) {
    init();
    for(int i = 0;i < n;i++) {
        if(same(edge[i][0], edge[i][1])) {
            cout << edge[i][0] << "" << edge[i][1] << endl;
            return;
        }else join(edge[i][0], edge[i][1]);
    }
}

int main() {
    cin >> n;
    vector<vector<int>> edge;
    vector<int> indgree(n + 1, 0);
    for(int i = 0;i < n;i++) {
        int s, t;
        cin >> s >> t;
        indgree[t]++;
        edge.push_back({s, t});
    }
    vector<int> vec;
    for(int i = n - 1;i >=0;i--) {
        if(indgree[edge[i][1]] == 2) {
            vec.push_back(i);
        }
    }
    if(vec.size() > 0) {
        if(isTree(edge, vec[0])) {
            cout << edge[vec[0]][0] << " " << edge[vec[0]][1] << endl;
        }else cout << edge[vec[1]][0] << " " << edge[vec[1]][1] << endl;
        return 0;
    }
    getremovedge(edge);
    return 0;
}