一本通1523：嗅探器

#include <bits/stdc++.h>

const int N = 2e5 + 5;

int n, m, s, t, tot, scc, ans = 1e9, top, child, dfn[N], low[N], stack[N];
bool haves[N], havet[N];
std::vector<int> edge[N];

void Dfs(int, int);
void Dfss(int);

signed main() {
    scanf("%d", &n);
    scanf("%d%d", &s, &t);

    while (s && t) {
        edge[s].push_back(t);
        edge[t].push_back(s);
        scanf("%d%d", &s, &t);
    }

    scanf("%d%d", &s, &t);
    haves[s] = true;
    havet[t] = true;

    for (int i = 1; i <= n; i++)
        if (!dfn[i]) {
            child = 0;
            Dfs(i, i);
        }

    if (ans == 1e9)
        printf("No solution");
    else
        printf("%d", ans);

    return 0;
}

void Dfs(int u, int fa) {
    dfn[u] = low[u] = ++tot;

    for (int v : edge[u]) {
        if (dfn[v]) {
            if (v != fa)
                low[u] = std::min(low[u], dfn[v]);

            continue;
        }

        Dfs(v, u);
        haves[u] |= haves[v];
        havet[u] |= havet[v];
        low[u] = std::min(low[u], low[v]);

        if (u == fa) {
            child++;

            if (child > 1 && (haves[v] ^ havet[v]) && u != s && u != t)
                ans = std::min(ans, u);
        } else if (low[v] >= dfn[u] && (haves[v] ^ havet[v]) && u != s && u != t)
            ans = std::min(ans, u);
    }
}