rt，交上去 WA #5 #6，其中有一个甚至显示了 "Too short on line 1."

#include<bits/stdc++.h>
using namespace std;
const int N = 2e5 + 5, M = 20;
int n, a, b, dfn[N], low[N], ts, stk[N], top, squ;
vector<int> tmp[N], v[N << 1];
void tarjan(int x, int fa) {
	dfn[x] = low[x] = ++ ts;
	stk[++ top] = x;
	for(int i : tmp[x]) {
		if(i == fa) {
			continue;
		}
		if(!dfn[i]) {
			tarjan(i, x);
			low[x] = min(low[x], low[i]);
			if(low[i] >= dfn[x]) {
				squ ++;
				while(top && stk[top] != x) {
					v[squ].push_back(stk[top]), v[stk[top]].push_back(squ);
					top --;
				}
				v[squ].push_back(x), v[x].push_back(squ);
			}
		} else {
			low[x] = min(low[x], dfn[i]);
		}
	}
}
void dfs(int x, int ans, int fa) {
	if(x == b) {
		if(ans < 0x3f3f3f3f / 2) {
			cout << ans << endl, exit(0);
		}
		return;
	}
	if(x <= n && v[x].size() > 1 && x != a) {
		ans = min(ans, x);
	}
	for(int i : v[x]) {
		if(i == fa) {
			continue;
		}
		dfs(i, ans, x);
	}
}
signed main() {
	cin >> n, squ = n;
	for(int x, y; ; ) {
		cin >> x >> y;
		if(!x && !y) {
			break;
		}
		tmp[x].push_back(y), tmp[y].push_back(x);
	}
	cin >> a >> b;
	tarjan(a, 0);
	dfs(a, 0x3f3f3f3f, 0);
	return cout << "No solution" << endl, 0;
}