Tunnels

#include <functional>
#include <algorithm>
#include <iostream>
#include <string>
#include <vector>
#include <cmath>
#include <queue>
#include <stack>
#include <list>
#include <set>
#include <map>
using namespace std;
#define rep(i,a,b) for(int i = (a); i < int(b); ++i)
#define rrep(i,a,b) for(int i = (b); i --> int(a); )
#define trav(i,v) for(auto&i:v)
#define all(c) (c).begin(), (c).end()
#define sz(c) int((c).size())
#define vcr(v,n) vi v(n); trav(var,v) cin >> var;
typedef long long ll;
typedef vector<int> vi;
typedef pair<int,int> pii;
int main(){
	cin.tie(0);
	cin.sync_with_stdio(0);
	int n,m;
	cin >> n >> m;
	vector<bool> used(m);
	vector<vector<pii>> in(n), out(n);
	vi inc(n),outc(n);
	rep(i,0,m){
		int u,v;
		cin >> u >> v;
		in[--v].push_back({--u,i});
		out[ u].push_back({  v,i});
		++inc[v], ++outc[u];
	}
	int ans = 0;
	function <void(int)> up = [&](int u){
		while(in[u].size() && used[in[u].back().second]) in[u].pop_back();
		if(in[u].empty()) return;
		pii e = in[u].back();
		used[e.second] = 1;
		in[u].pop_back();
		--inc[u], --outc[e.first];
		up(e.first);
	}, dn = [&](int u){
		while(out[u].size() && used[out[u].back().second]) out[u].pop_back();
		if(out[u].empty()) return;
		pii e = out[u].back();
		used[e.second] = 1;
		out[u].pop_back();
		--outc[u], --inc[e.first];
		dn(e.first);
	};
	rep(i,0,n) while(inc[i]+outc[i])
		up(i),dn(i),++ans;
	cout << ans << endl;
}

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