Running In Circles

    1#include <bits/stdc++.h>
    2using namespace std;
    3vector<vector<int>> adj;
    4vector<int> color, depth, parent;
    5int ans = -1;
    6void dfs(int u){
    7    color[u] = 1; // visiting
    8    for (int v : adj[u]){
    9        if (ans != -1) return; // cycle already found
    10        if (color[v] == 0){
    11            parent[v] = u;
    12            depth[v] = depth[u] + 1;
    13            dfs(v);
    14        } else if (color[v] == 1){
    15            ans = depth[u] - depth[v] + 1; // foudn unique cycle
    16            return;
    17        }
    18    }
    19    color[u] = 2; // done
    20}
    21 
    22int main(){
    23    ios::sync_with_stdio(0);
    24    cin.tie(0);
    25    for (int tc = 0; tc < 5; tc++){
    26        int m; cin >> m;
    27        adj.assign(101, {});
    28        color.assign(101, 0);
    29        depth.assign(101, 0);
    30        parent.assign(101, -1);
    31        ans = -1;
    32        int start = -1;
    33        for (int i = 0; i < m; i++){
    34            int u, v; cin >> u >> v;
    35            if (i == 0) start = u;
    36            adj[u].push_back(v);
    37        }
    38        dfs(start);
    39        cout << ans << "\n";
    40    }
    41    return 0;
    42}
    43