Description

Input

Output

一个整数即食物网中的食物链条数

Sample Input

Sample Output

DP水题

#include<iostream>
#include<cstdio>
#include<cstdlib>
#include<cmath>
#include<cstring>
#include<algorithm>
#define F(i,j,n) for(int i=j;i<=n;i++)
#define D(i,j,n) for(int i=j;i>=n;i--)
#define ll long long
#define N 100005
using namespace std;
int n,m,cnt,ans,f[N],head[N],in[N];
bool tag[N];
struct edge{int next,to;}e[N*2];
inline int read()
{
	int x=0,f=1;char ch=getchar();
	while (ch<'0'||ch>'9'){if (ch=='-') f=-1;ch=getchar();}
	while (ch>='0'&&ch<='9'){x=x*10+ch-'0';ch=getchar();}
	return x*f;
}
inline void add_edge(int x,int y){e[++cnt]=(edge){head[x],y};head[x]=cnt;}
void dfs(int x)
{
	for(int i=head[x];i;i=e[i].next)
	{
		int y=e[i].to;
		f[y]+=f[x];in[y]--;
		if (!in[y]) dfs(y);
	}
}
int main()
{
	n=read();m=read();
	F(i,1,m){int x=read(),y=read();add_edge(x,y);in[y]++;}
	F(i,1,n) tag[i]=!in[i]&&head[i];
	F(i,1,n) if (tag[i]) f[i]=1,dfs(i);
	F(i,1,n) if (!head[i]) ans+=f[i];
	printf("%d\n",ans);
	return 0;
}

顶

0

踩

0