Tunnels

#include<bits/stdc++.h>

#define F first
#define S second
#define ll long long
#define ull unsigned long long
#define pb push_back
using namespace std;
ll n, m, used[200000], r, d[200000], c, s;
vector < ll > k;
set < ll > g[200000], gg, dif;
ll dp[200000], f[200000], b[200000], ans;
int dfs(int v){
    if (dp[v] != 0) return dp[v];
    for (auto u: g[v]){
        if (dfs(u) + 1 > dp[v]){
            f[v] = u;
            dp[v] = dfs(u) + 1;
        }
    }
    dif.insert(v);
    return dp[v];
}
void dfs1(int v)
{
    if (f[v] == -1) return;
    g[v].erase(f[v]);
    b[f[v]]--;
    if (b[f[v]] == 0 && f[v] != -1) gg.insert(f[v]);
    dfs1(f[v]);
}
int main()
{
    ios_base::sync_with_stdio(false);
    cin.tie(NULL);
    cin >> n >> m;
    for (int i = 1; i <= m; i++){
        int u, v;
        cin >> u >> v;
        g[u].insert(v);
        b[v]++;
    }
    for (int i = 1; i <= n; i++){
        if (b[i] == 0) gg.insert(i);
        dp[i] = 0;
        f[i] = -1;
    }
    while(!gg.empty()){

        int i = *gg.begin();
        if (g[i].size() == 0)
        gg.erase(i);
        dfs(i);
        dfs1(i);
        if (f[i] != -1)
        ans++;
        for(auto u: dif){
            dp[u] = 0;
            f[u] = -1;
        }
        dif.clear();
    }
    cout << ans;
}

Test 1

Group: 1

Verdict: ACCEPTED

Test 2

Group: 1

Verdict: ACCEPTED

Test 3

Group: 1

Verdict: ACCEPTED

Test 4

Group: 1

Verdict: ACCEPTED

Test 5

Group: 1

Verdict: ACCEPTED

Test 6

Group: 2

Verdict: ACCEPTED

Test 7

Group: 2

Verdict: ACCEPTED

Test 8

Group: 2

Verdict: ACCEPTED

Test 9

Group: 2

Verdict: ACCEPTED

Test 10

Group: 2

Verdict: ACCEPTED

Test 11

Group: 3

Verdict: ACCEPTED

Test 12

Group: 3

Verdict: ACCEPTED

Test 13

Group: 3

Verdict: ACCEPTED

Test 14

Group: 3

Verdict: ACCEPTED

Test 15

Group: 3

Verdict: ACCEPTED