Dinic 算法

#include <bits/stdc++.h>
using namespace std;

const int N (1e5+5), M(1e5+5);

int head[N];
struct Edge{
    /*
    r: residual capacity
    */
    int v, r, nt;
}E[M];
int tail;
void add_edge(int u, int v, int c){
    E[tail]={v, c, head[u]}, head[u]=tail++;
    E[tail]={u, 0, head[v]}, head[v]=tail++;
}

int level[N];
void bfs(int s){
    memset(level, -1, sizeof(level));
    queue<int> que;
    level[s]=0;
    que.push(s);
    for(int u; !que.empty(); ){
        u=que.front(), que.pop();
        for(int i=head[u]; ~i; i=E[i].nt){
            int &v=E[i].v;
            if(E[i].r>0 && level[v]<0){
                level[v]=level[u]+1;
                que.push(v);
            }
        }
    }
}

int iter[N];
int dfs(int u, int t, int f){
    /*
    t: terminal (sink)
    */
    if(u==t) return f;
    //for(int &i=iter[u]; i!=-1; i++){
    for(int &i=iter[u]; i!=-1; i=E[i].nt){
        int &v=E[i].v, &r=E[i].r;
        if(r>0 && level[u]<level[v]){
            int d=dfs(v, t, min(f, r));
            if(d>0){
                r-=d;
                E[i^1].r+=d;
                return d;
            }
        }
    }
    return 0;
}

int dinic(int s, int t){
    const int INF=1<<30;
    for(int flow=0;;){
        bfs(s);
        if(level[t]<0) return flow;
        memcpy(iter, head, sizeof(iter));
        for(int f; f=dfs(s, t, INF); flow+=f);
    } 
}

void init(){
    tail=0;
    memset(head, -1, sizeof(head));
}