ISAP

2171. EK求最大流

#include <bits/stdc++.h>
using namespace std;
const int N=1e4+5;
const int M=1e6+5;

int h[N],ne[M],e[M],w[M],tot=1;
void add(int from,int to,int wi) {
    e[++tot]=to;
    w[tot]=wi;
    ne[tot]=h[from];
    h[from]=tot;
}

int n,m,S,T;
int dep[N],gap[N];
void bfs() {
    dep[T]=1;
    queue<int>q;
    q.push(T);
    while(!q.empty()) {
        int now=q.front();
        gap[dep[now]]++;//统计深度的个数
        q.pop();
        for(int i=h[now];i;i=ne[i]) {
            int to=e[i];
            if(dep[to])continue;
            q.push(to);
            dep[to]=dep[now]+1;
        }
    }
}

int cur[N];
int dfs(int now,int sum) {
    if(now==T)return sum;
    int ans=0;
    for(int i=cur[now];i&&sum;i=ne[i]) {
        cur[now]=i;
        int to=e[i];
        if(w[i]>0&&dep[now]==dep[to]+1) {
            int k=dfs(to,min(sum,w[i]));
            ans+=k;
            sum-=k;
            w[i]-=k;
            w[i^1]+=k;
        }
    }
    if(sum==0)return ans;
    //流不完
    --gap[dep[now]];
    if(gap[dep[now]]==0)dep[S]=n+1;
    dep[now]++;
    gap[dep[now]]++;
    return ans;
}

int ISAP() {
    int ans=0;
    bfs();
    while(dep[S]<=n) {
        memcpy(cur,h,sizeof(h));
        ans+=dfs(S,1e9);
    }
    return ans;
}

int main() {
    cin>>n>>m>>S>>T;
    while(m--) {
        int x,y,wi;
        cin>>x>>y>>wi;
        add(x,y,wi);
        add(y,x,0);
    }
    cout<<ISAP()<<endl;
    return 0;
}