51nod1076 2条不相交的路径

在环路上的两点一定可以，否则就不行。
用tarjan算法就可以解决

#include<cstdio>
#include<vector>
#include<algorithm>
#include<stack>
using namespace std;

const int maxn = 50500;
vector<int> a[maxn];
int dfn[maxn];
int low[maxn];
int vis[maxn];
int f[maxn];
stack<int> s;
int tot = 1;

void dfs(int k, int fa){
    dfn[k] = low[k] = tot++;
    if(a[k].size() == 1) return;
    s.push(k);
    vis[k] = 1;
    for(int i = 0; i < a[k].size(); i++){
        int t = a[k][i];
        if(t == fa) continue;
        if(dfn[t] == 0){
            dfs(t, k);
            low[k] = min(low[k], low[t]);
        }else if(vis[t] == 1){
            low[k] = min(low[k], dfn[t]);
        }
    }
    if(low[k] == dfn[k]){
        while(!s.empty() && s.top() != k){
            vis[s.top()] = 0;
            low[s.top()] = k;
            s.pop();
        }
        if(!s.empty()) {
            vis[s.top()] = 0;
            s.pop();
        }
    }
}

int main(){
    int n, m, k;
    scanf("%d%d", &n, &m);
    for(int i = 1; i <= m; i++){
        int x, y;
        scanf("%d%d", &x, &y);
        a[x].push_back(y);
        a[y].push_back(x);
    }
    for(int i = 1; i <= n; i++){
        if(dfn[i] == 0){
            dfs(i, i);
        }
    }
    scanf("%d", &k);
    for(int i = 1; i <= k; i++){
        int x, y;
        scanf("%d%d", &x, &y);
        if(low[x] == low[y]) puts("YES");
        else puts("NO");
    }
    return 0;
}