Codeforces 1006E

#include<bits/stdc++.h>
using namespace std;
const int maxn = ;

int dfn[maxn],rdfn[maxn],children[maxn];
vector<int> graph[maxn];
int n,q,cnt;

int dfs(int u){
    int ret = ;
    dfn[u] = cnt;
    rdfn[cnt++] = u;
    for(int i = ;i < graph[u].size();++i){
        int to = graph[u][i];
        ret += dfs(to);
    }
    return children[u] = ret;
}

int main(){
    scanf("%d%d",&n,&q);
    int temp;
    for(int i = ;i <= n;++i){
        scanf("%d",&temp);
        graph[temp].push_back(i);
    }
    children[] = dfs();
    int v,numb;
    for(int i = ;i < q;++i){
        scanf("%d%d",&v,&numb);
        if(children[v] < numb) printf("-1\n");
        else printf("%d\n",rdfn[dfn[v] + numb - ]);
    }
    return ;
}

dfs序的使用。。。。