1160 - Planets Queries II.cpp

/*
Problem Name: Planets Queries II
Problem Link: https://cses.fi/problemset/task/1160
Author: Sachin Srivastava (mrsac7)
*/
#include<bits/stdc++.h>
using namespace std;

template<typename... T>
#define error(args...) { string _s = #args; replace(_s.begin(), _s.end(), ',', ' '); stringstream _ss(_s); istream_iterator<string> _it(_ss); err(_it, args); }
void err(istream_iterator<string> it) {}
template<typename T, typename... Args>
void err(istream_iterator<string> it, T a, Args... args) {cerr << *it << "=" << a << ", "; err(++it, args...);}

#define int long long
#define ff first
#define ss second
#define endl '\n'

const long long inf = 1LL<<60; //1.5e18
const int md = 1000000007;

int succ[21][200005];
int vis[200005];
int len[200005];
void dfs(int s) {
    if (vis[s]) return;
    vis[s] = 1;
    dfs(succ[0][s]);
    len[s] = len[succ[0][s]] + 1;
}
int lift(int x, int d) {
    if (d <=0) return x;
    int i = 0;
    while(d > 0) {
        if (d&1) 
            x = succ[i][x];
        d>>=1;
        i++;
    }
    return x;
}
void solve(){
    int n,q; cin>>n>>q;
    for (int i = 1; i <= n; i++)
        cin>>succ[0][i];
    for (int i = 1; i <=20; i++) {
        for (int j = 1; j <=n; j++) {
            succ[i][j] = succ[i-1][succ[i-1][j]];
        }
    }
    for  (int i = 1; i <=n; i++)
        if (!vis[i])
            dfs(i);
    while(q--) {
        int x,y; cin>>x>>y;
        int xx = lift(x, len[x]); //end of the cycle (Case: x and y are in cycle such that x appears after y)
        if (lift(x, len[x]-len[y]) == y) //if y is infront of x
            cout<<len[x]-len[y]<<endl;
        else if (lift(xx, len[xx]-len[y]) == y) 
            cout<<len[x]+len[xx]-len[y]<<endl;
        else 
            cout<<-1<<endl;
    }
}    
signed main(){
    ios_base::sync_with_stdio(false);cin.tie(0);cout.tie(0);
    #ifdef LOCAL
    freopen("input.txt", "r" , stdin);
    freopen("output.txt", "w", stdout);
    #endif
    int t=1;
    //cin>>t;
    for (int i = 1; i <= t; i++) {
        //cout<<"Case #"<<i<<": ";
        solve();
        cout<<'\n';
    }
}