python/TangDou/AcWing/Test/Test.cpp

#include <bits/stdc++.h>

using namespace std;
const int N = 100010;
int b[N], in[N];

int main() {
    int n, m;
    cin >> n >> m;
    for (int i = 1; i <= n; i++) {
        int x, y;
        cin >> x >> y;
        in[y]++;
        b[x] = y;
    }
    int head = 0;
    for (int i = 1; i <= n; i++)
        if (in[i] == 0) {
            head = i;
            break;
        }
    for (int i = 1; i <= m; i++) {
        int x, y;
        cin >> x >> y;
        int t = b[x];
        b[x] = y;
        b[y] = t;
    }
    cout << head << endl;
    int x = b[head];
    while(x){
        cout << x << endl;
        x = b[x];
    }
    return 0;
}