讨论区的hack都过了

#include <bits/stdc++.h>
using namespace std;
const int N = 4e5 + 5;
const int inf = 0x3f3f3f3f;
int n;
int cnt;
vector<int> g[N];
vector<int> h[N];
int low[N];
int dfn[N];
int dfs_clock;
int ans = 0x3f3f3f3f;
int st, en;
stack<int> S;

void tarjan(int u, int fa) {
    int child = 0;
    low[u] = dfn[u] = ++dfs_clock;
    S.push(u);
    for (int v : g[u]) {
        if (!dfn[v]) {
            tarjan(v, u);
            low[u] = min(low[u], low[v]);
            if (low[v] >= dfn[u]) {
                cnt++;
                while (true) {
                    int x = S.top();
                    h[cnt].push_back(x);
                    h[x].push_back(cnt);
                    if (x == u) break;
                    S.pop();
                }
            }
        } else if (v != fa) {
            low[u] = min(low[u], dfn[v]);
        }
    }
}
void dfs(int u, int fa, int ans) {
    if (u == en) {
        if (ans == inf) cout << "No solution";
        else cout << ans;
        return ;
    }
    for (int v : h[u]) {
        if (v != fa) {
            if (u != st && u <= n) ans = min(ans, u);
            dfs(v, u, ans);
        }
    }
    
}
int main() {
    cin >> n;
    int u, v;
    cnt = n;
    while (cin >> u >> v) {
        if (u == 0 && v == 0) break;
        g[u].push_back(v);
        g[v].push_back(u);
    }
    cin >> st >> en;
    for (int i = 1; i <= n; i++) if (!dfn[i]) tarjan(i, 0);
    dfs(st, 0, inf);
    
    return 0;
}