题解-空间之刀

#include<bits/stdc++.h>
using namespace std;

bool flag, vis[100];
vector<int> a[100];

void dfs(int n, int target) {
    vis[n] = 1;
    if (n == target) {
        flag = true;
        return;
    }
    for (int i = 0; i < a[n].size(); i++) {
        int next = a[n][i];
        if (!vis[next]) {
            dfs(next, target);
            if (flag) return;
        }
    }
}

int main() {
    int n; cin >> n;
    for (int i = 1; i < n; i++) {
        int x, y;
        cin >> x >> y;
        a[x-1].push_back(y-1); // 统一存储为 0-based
    }

    for (int target = 0; target < n; target++) {
        bool ok = true;
        for (int start = 0; start < n; start++) {
            if (start == target) continue;
            memset(vis, 0, sizeof vis);
            flag = false;
            dfs(start, target);
            if (!flag) {
                ok = false;
                break;
            }
        }
        if (ok) {
            cout << target + 1 << endl;
            return 0;
        }
    }
    cout << -1 << endl;
    return 0;
}

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