CodeForces - 1006E Military Problem （ DFS

题意：

#include <algorithm>
#include <cstdio>
#include <cstring>
#include <iostream>
#include <bits/stdc++.h>
using namespace std;

#define pb push_back
#define ll long long
const int MAXN = 2e5+10;

int in[MAXN], out[MAXN];
int ans[MAXN];
vector<int> G[MAXN];
int tot;
void dfs(int u) {
    in[u] = ++tot;
    ans[tot] = u;
    for(int i = 0; i < G[u].size(); ++i) {
        int v = G[u][i];
        dfs(v);
    }
    out[u] = tot;
}
int main() {
    #ifndef ONLINE_JUDGE
        freopen("in.txt","r",stdin);
    #endif // ONLINE_JUDGE
    ios::sync_with_stdio(false);
    tot = 0;
    int n, q, x;
    cin >> n >> q;
    for(int i = 2; i <= n; i++) {
        cin >> x;
        G[x].pb(i);
    }
    dfs(1);
    while(q--) {
        int u, k;
        cin >> u >> k;
        if(out[u]-in[u]+1 < k)
            cout << "-1\n";
        else
            cout << ans[in[u]+k-1] << endl;
    }
    return 0;
}