hihoCoder 网络流四·最小路径覆盖

#include<cstdio>
#include<iostream>
#include<algorithm>
#include<cstring>
#include<queue>
using namespace std;
struct node
{
    int point;
    int weight;
    int nxt;
};
node line[5000000];
int head[50000],tail=-1;
void add(int x,int y,int z)
{
    line[++tail].point=y;
    line[tail].weight=z;
    line[tail].nxt=head[x];
    head[x]=tail;
    
    line[++tail].point=x;
    line[tail].weight=0;
    line[tail].nxt=head[y];
    head[y]=tail;
}
int cur[50000],dep[50000];
bool BFS(int begin,int end)
{
    for(int i=begin;i<=end;i++)
    {
        cur[i]=head[i];
        dep[i]=0;
    }
    dep[begin]=1;
    queue<int>q;
    q.push(begin);
    while(!q.empty())
    {
        int pas=q.front();
        q.pop();
        for(int i=head[pas];i!=-1;i=line[i].nxt)
            if(!dep[line[i].point]&&line[i].weight)
            {
                dep[line[i].point]=dep[pas]+1;
                q.push(line[i].point);
            }
    }
    if(dep[end])
        return true;
    return false;
}
int DFS(int now,int aim,int limte)
{
    if(now==aim||!limte)
        return limte;
    int flow=0,f;
    for(int i=cur[now];i!=-1;i=line[i].nxt)
    {
        cur[now]=i;
        if(dep[line[i].point]==dep[now]+1&&(f=DFS(line[i].point,aim,min(limte,line[i].weight))))
        {
            limte-=f;
            flow+=f;
            line[i].weight-=f;
            line[i^1].weight+=f;
            if(!limte)
                break;
        }
    }
    return flow;
}
int dinic(int begin,int end)
{
    int res=0;
    while(BFS(begin,end))
        res+=DFS(begin,end,0x7fffffff);
    return res;
}//以上都是标准的dinic。ISAP就是个垃圾
int main()
{
    int n,m;
    scanf("%d%d",&n,&m);
    for(int i=0;i<=2*n+1;i++)//0为起点，2*n+1为终点
        head[i]=-1;
    int a,b;
    for(int i=1;i<=m;i++)
    {
        scanf("%d%d",&a,&b);
        add(a,b+n,1);
    }
    for(int i=1;i<=n;i++)
        add(0,i,1);
    for(int i=1;i<=n;i++)
        add(n+i,2*n+1,1);
    int ans=dinic(0,2*n+1);
    printf("%d",n-ans);
}