【Codeforces】 489D Unbearable Controversy of Being

#include <iostream>
#include <queue>
#include <stack>
#include <map>
#include <set>
#include <bitset>
#include <cstdio>
#include <algorithm>
#include <cstring>
#include <climits>
#include <cstdlib>
#include <cmath>
#include <time.h>
#define maxn 3005
#define maxm 300005
#define eps 1e-10
#define mod 1000000007
#define INF 0x3f3f3f3f
#define PI (acos(-1.0))
#define lowbit(x) (x&(-x))
#define mp make_pair
#define ls o<<1
#define rs o<<1 | 1
#define lson o<<1, L, mid
#define rson o<<1 | 1, mid+1, R
//#pragma comment(linker, "/STACK:16777216")
typedef long long LL;
typedef unsigned long long ULL;
//typedef int LL;
using namespace std;
LL qpow(LL a, LL b){LL res=1,base=a;while(b){if(b%2)res=res*base;base=base*base;b/=2;}return res;}
LL powmod(LL a, LL b){LL res=1,base=a;while(b){if(b%2)res=res*base%mod;base=base*base%mod;b/=2;}return res;}
void scanf(int &__x){__x=0;char __ch=getchar();while(__ch==' '||__ch=='\n')__ch=getchar();while(__ch>='0'&&__ch<='9')__x=__x*10+__ch-'0',__ch = getchar();}
LL gcd(LL _a, LL _b){if(!_b) return _a;else return gcd(_b, _a%_b);}
// head

struct Edge
{
int v;
Edge *next;
}E[maxm], *H[maxn], *edges;

int cnt[maxn];
int vis[maxn];
int T;
int n, m;
LL ans;

void addedges(int u, int v)
{
edges->v = v;
edges->next = H[u];
H[u] = edges++;
}

void init(void)
{
ans = T = 0;
edges = E;
memset(H, 0, sizeof H);
memset(vis, 0, sizeof vis);
}

void read(void)
{
int u, v;
scanf("%d%d", &n, &m);
while(m--) {
scanf("%d%d", &u, &v);
addedges(u, v);
}
}

void dfs(int u, int dep, int fa)
{
for(Edge *e = H[u]; e; e = e->next) {
int v = e->v;
if(v == fa) continue;
if(dep) {
if(vis[v] != T) vis[v] = T, cnt[v] = 1;
else ans += cnt[v], cnt[v]++;
}
else dfs(v, 1, u);
}
}

void work(void)
{
for(int i = 1; i <= n; i++) {
++T;
vis[i] = T;
dfs(i, 0, i);
}
printf("%I64d\n", ans);
}

int main(void)
{
init();
read();
work();

return 0;
}