algorithm_2024/P2661/main.cpp

#include <algorithm>
#include <iostream>
#include <istream>
#include <queue>
#include <vector>
#include <functional>

using std::cin,std::cout;

const int MAX_N {(int)2e5+5};
int n;
std::vector<int> next[MAX_N];
int in_degree[MAX_N];
bool vis[MAX_N];
std::queue<int> q;
int ans;
int input;

int dfs(const int start, const int now, int depth){
    depth++;
    vis[now] = true;
    int max_depth{ depth };
    for (int i : next[now]) {
        if (i == start) {
            max_depth = std::max(max_depth, depth);
        }
        else {
            max_depth = std::max(dfs(start, i, depth), max_depth);
        }
    }
    return max_depth;
};

int main(){
    std::iostream::sync_with_stdio(false),cin.tie(nullptr),cout.tie(nullptr);

    std::cin>>n;
    for(int i=1;i<=n;i++){
        cin>>input;
        in_degree[input]++;
        next[i].push_back(input);
    }
    const auto push_vis = [](int n){
        q.push(n);
        vis[n]=true;
    };
    for(int i=1;i<=n;i++){
        if(in_degree[i]<=0){
            push_vis(i);
        }
    }
    while(q.empty()==false){
        int front = q.front();
        q.pop();
        for(int i:next[front]){
            if(--in_degree[i]<=0){
                push_vis(i);
            }
        }
        
    }
    
    for(int i=1;i<=n;i++){
        if(vis[i])continue;
        ans=std::max(ans,dfs(i,i,0));
    }
    cout<<ans<<"\n";
}