网络最大流算法—最高标号预流推进HLPP

#include<iostream>
#include<cstdio>
#include<cstring>
#include<cmath>
#include<queue>
using namespace std;
const int MAXN=2*1e3+10;
const int INF=1e8+10;
inline char nc()
{
    static char buf[MAXN],*p1=buf,*p2=buf;
    return p1==p2&&(p2=(p1=buf)+fread(buf,1,MAXN,stdin),p1==p2)?EOF:*p1++;
}
inline int read()
{
    char c=nc();int x=0,f=1;
    while(c<'0'||c>'9'){if(c=='-')f=-1;c=nc();}
    while(c>='0'&&c<='9'){x=x*10+c-'0';c=nc();}
    return x*f;
}
int N,M,S,T;
int H[MAXN];//每个节点的高度
int F[MAXN];//每个节点可以流出的流量
int gap[MAXN];//每个高度的数量 
struct node
{
    int u,v,flow,nxt;
}edge[MAXN];
int head[MAXN];
int num=0;//注意这里num必须从0开始 
inline void add_edge(int x,int y,int z)
{
    edge[num].u=x;
    edge[num].v=y;
    edge[num].flow=z;
    edge[num].nxt=head[x];
    head[x]=num++;
}
inline void AddEdge(int x,int y,int z)
{
    add_edge(x,y,z);
    add_edge(y,x,0);//注意这里别忘了加反向边 
}
struct comp
{
    int pos,h;
    comp(int pos=0,int h=0):pos(pos),h(h) {}
    inline bool operator < (const comp &a) const {return h<a.h;} 
};
priority_queue<comp>q;
bool Work(int u,int v,int id)
{
    int val=min(F[u],edge[id].flow);
    edge[id].flow-=val;edge[id^1].flow+=val;
    F[u]-=val;F[v]+=val;
    return val;
}
inline int HLPP()
{
    H[S]=N;F[S]=INF;q.push(comp(S,H[S]));
    while(q.size()!=0)
    {
        int p=q.top().pos;q.pop();
        if(!F[p]) continue;
        for(int i=head[p];i!=-1;i=edge[i].nxt)
            if( (p==S||H[edge[i].v]+1==H[p]) && Work(p,edge[i].v,i) && edge[i].v!=S && edge[i].v!=T)
                q.push( comp(edge[i].v,H[edge[i].v]) );
        if(p!=S && p!=T && F[p])
        {
            if( (--gap[ H[p] ])==0 )//该高度不存在 
            {
                for(int i=1;i<=N;i++)
                    if( H[p]<H[i]&&H[i]<=N && p!=S && p!=T ) 
                        H[i]=N+1;//设置为不可访问 
            }
            ++gap[ ++H[p] ];//高度+1 
            q.push( comp(p,H[p]) );
        }
    }
    return F[T];
}
int main()
{
    #ifdef WIN32
    freopen("a.in","r",stdin);
    #else
    #endif 
    memset(head,-1,sizeof(head));
    N=read(),M=read(),S=read(),T=read();
    for(int i=1;i<=M;i++)
    {
        int x=read(),y=read(),z=read();
        AddEdge(x,y,z); 
    }
    printf("%d", HLPP() ); 
    return 0;
}