裸的DFS
比赛的时候没想出来 血亏 后面用stack写了个
https://pintia.cn/problem-sets/994805046380707840/problems/994805055679479808

#include <stdio.h>
#include <iostream>
#include <cstring>
#include <vector>
#include <queue>
#include <stack>
#include <set>
#include <sstream>
#include <algorithm>
int N, M;
//const int MAXN = , MAXM = 0;
//typedef long long ll;
const int si =100000;
using namespace std;
int par[si];
int rankk[si];

vector<int> v;
stack<int> stk;
int maxx = 0;

void solve(int rt) {
    int p = rt;
    
    while (!rankk[p]) {
        stk.push(p);
        p = par[p];
    }

    int it = rankk[p];
    int x = 1;
    while (!stk.empty()) {
        x = stk.top(); stk.pop();
        rankk[x] = ++it;
    }
    maxx = max(rankk[x], maxx);
}
int main() {
    cin >> N;
    int rt = 0;
    for (int i = 1; i <= N; i++) {
        scanf("%d", &par[i]);
        if (par[i] == -1) rt = i;
    }
    
    //solve
    rankk[rt] = 1;
    for (int i = 1; i <= N; i++) {
        solve(i);
    }
    

        //output
    for (int i = 1; i <= N; i++) {
        if (rankk[i] == maxx)
            v.push_back(i);
    }
    cout << maxx << endl;
    for (int i = 0; i < v.size() - 1; i++) {
        printf("%d ", v[i]);
    }
    printf("%d", v[v.size() - 1]);
    cout << endl;
    return 0;
}