在基环树上 判断一个点到另外一个点的路径是不是大于2

树：n点 n-1边
基环树：n点 n以上边

#include <bits/stdc++.h>

using namespace std;
const int N = 2e5 + 5, M = N*2;
int n, q;
int h[N], e[M], ne[M], idx;
int fa[N], d[N];

void add (int a, int b) {
    e[idx] = b, ne[idx] = h[a], h[a] = idx ++;
}

void topsort () {
    queue <int> q;
    for (int i = 1; i <= n; i++) {
        if (d[i] == 1)  q.push (i);
    }

    while (!q.empty()) {
        int t = q.front();
        q.pop ();
        fa[t] = -1; //无环 因为度数为1

        for (int i = h[t]; ~i; i = ne[i]) {
            int j = e[i];
            if (--d[j] == 1)    q.push (j);
        }
    }
}

void dfs (int u, int v) {
    fa[u] = v;
    for (int i = h[u]; ~i; i = ne[i]) {
        int j = e[i];
        if (fa[j] == -1)    dfs (j, v);//如果是不在环上的点 就把一条链上的点都设置为在环上的点
    }
}

int main () {
    memset (h, -1, sizeof h);
    cin >> n;

    for (int i = 0; i < n; i++) {
        int a, b;
        cin >> a >> b;
        add (a, b), add (b, a);
        d[a] ++, d[b] ++;
    }

    //topsort找环 不在环上会标记成-1
    topsort ();
    for (int i = 1; i <= n; i++) {
        if (fa[i] == 0) dfs (i, i);//剩下的就是在环上的了
    }

    cin >> q;
    while (q --) {
        int u, v;
        cin >> u >> v;
        if (fa[u] == fa[v])     cout << "Yes\n";
        else    cout << "No\n";
    }
}


//dfs合并到环上

标签：基环,一个点,idx,fa,int,路径,++,add,topsort
来源： https://www.cnblogs.com/liang302/p/16635629.html