Codeforces Round #357 (Div. 2) D. Gifts by the List DFS

D. Gifts by the List

链接：

http://codeforces.com/contest/681/problem/D

代码：

#include<iostream>
#include<vector>
using namespace std;

const int maxn = 1e6 + 7;
int a[maxn], d[maxn], flag;
vector<int> E[maxn];
vector<int> ans;

void dfs(int x, int p)
{
    for (int i = 0; i < E[x].size(); i++)
    {
        int u = E[x][i];
        if (u == p) continue;
        if (a[u] != a[x] && a[u] != u) flag = 1;
        dfs(u, x);
    }
    if (a[x] == x) ans.push_back(x);
}

int main()
{
    int n, m;
    cin >> n >> m;
    for (int i = 1; i <= m; i++) {
        int p, q;
        cin >> p >> q;
        E[p].push_back(q);
        E[q].push_back(p);
        d[q]++;
    }
    for (int i = 1; i <= n; i++)
        cin >> a[i];
    for (int i = 1; i <= n; i++)
        if (!d[i])
            dfs(i, 0);
    if (flag) {
        cout << -1 << endl;
        return 0;
    }
    cout << ans.size() << endl;
    for (int i = 0; i < ans.size(); i++)
        cout << ans[i] << endl;
    return 0;
}