CPPCodeSnippets/binaryLifting.cpp at master · rnagasudhakar/CPPCodeSnippets · GitHub

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#include<bits/stdc++.h>
using namespace std;
#define ll long long

vector<vector<int>>dp(200001,vector<int>(20));


void precompute(){
	int k = ceil(log2(x));
	for(int j=1;j<=k;j++){
		for(int i=0;i<x;i++){
			int par=dp[i][j-1];
			if(par!=-1){
				dp[i][j]=dp[par][j-1];/
			}

		}
	}
}

void dfs(vector<vector<int>> &graph, int sou, int par, int level, vector<int>&depth){
	dp[sou][0]=par;
	depth[sou]=level;
	for(auto it:graph[sou]){
		if(it!=par){
			dfs(graph,it,sou,level+1,depth);
		}
	}
}


int main(){
	int x,y;
	cin>>x>>y;
	vector<int>v(x);
	for(int i=0;i<x-1;i++){
		cin>>v[i];
	}
	vector<vector<int>>graph(x);
	for(int i=0;i<x-1;i++){
		graph[v[i]-1].push_back(i+1);
		graph[i+1].push_back(v[i]-1);
	}

	while(y--){
		int a, b;
		cin>>a>>b;
		a--;
		if(depth[a]<b){
			cout<<-1<<endl;
			continue;
		}
		while(b>0){
			int c = log2(b);
			b=dp[b][c];
			b-=1<<c;
		}

	}


	return 0;
}