Hair candy (topological sorting template)

Hair candy

Code

#include <iostream>
#include <cstdio>
#include <cstring>
#include <cmath>
#include <cstdlib>
#include <algorithm>
#include <stack>
using namespace std;
const int N = 1e5 + 3;
int n, m, buy[N], in[N];
long long sum;
struct edge {
    int to, next;
}e[N << 1];
int head[N], cnt;
stack <int> s;
void add(int u,int v)
{
    e[++cnt].next = head[u];
    e[cnt].to = v;
    head[u] = cnt;
}
int main()
{
    cin >> n >> m;
    for(int i = 1; i <= m; i++) {
        int u, v;
        cin >> u >> v;
        add(v, u);
        in[u]++;
    }
    cnt = 0;
    for(int i = 1; i <= n; i++) {
        if(in[i] == 0) {
            s.push(i);
            buy[i] = 1;
        }
    }
    while(!s.empty()) {
        int u = s.top();
        s.pop();
        cnt++;
        for(int i = head[u]; i; i = e[i].next) {
            int v = e[i].to;
            in[v]--;
            if(in[v] == 0) {
                s.push(v);  
            }
            buy[v] = max(buy[v], buy[u] + 1);
        }
    }
    if(cnt != n) {
        cout << -1 << endl;
    } else {
        for(int i = 1; i <= n; i++) {
            sum += buy[i];
        }
        cout << sum << endl;
    }
    return 0;
}

Issues Note

e[++cnt].next = head[u];
e[cnt].to = v;
head[u] = cnt;

Labeled

e[cnt++].next = head[u];
e[cnt].to = v;
head[u] = cnt;