網路最大流

DInic

#include <cstdio>
#include <cstring>
#include <algorithm>
#define MAXN 10005
#define MAXM 100005
#define INF 2147483647

struct queue {
    int q[MAXN]; int head,tail;
    queue() : head(1),tail(0) {}
    void reset() {
        head = 1; tail = 0;
    }
    void push(int x) {
        q[++tail] = x;
    }
    int front() {
        return q[head];
    }
    void pop() {
        ++head;
    }
    bool empty() {
        return head>tail;
    }
}q;

struct edge {
    int v,cap,next;
}G[MAXM<<1];

int head[MAXN],cur[MAXN],step[MAXN];
int ans = 0,tot = -1;
int N,M,S,T;

inline bool bfs() {

    q.reset(); q.push(S);
    std::memset(step,-1,sizeof(step));
    step[S] = 1;

    while(!q.empty()) {
        int u = q.front(); q.pop();
        for(int i=head[u];i!=-1;i=G[i].next) {
            int v = G[i].v;
            if(G[i].cap>0&&step[v]==-1) {
                q.push(v);
                step[v] = step[u] + 1;
            }
        }
    }
    return step[T]!=-1;
}

int dfs(int u,int a) {
    if(u==T) return a;
    int flow = 0,temp;
    for(int& i=cur[u];i!=-1;i=G[i].next) {
        int v = G[i].v;
        if(step[u]+1==step[v]&&G[i].cap&&(temp = dfs(v,std::min(a,G[i].cap)))>0) {
            flow += temp; a -= temp; 
            G[i].cap -= temp; G[i^1].cap += temp;
            if(a==0) break;
        }
    }
    return flow;
}

inline void dinic() {
    while(bfs()) {
        for(int i=1;i<=N;++i) cur[i] = head[i];
        ans += dfs(S,INF);
    }
}

inline void add(int u,int v,int cap) {
    G[++tot].v = v; G[tot].cap = cap; G[tot].next = head[u]; head[u] = tot;
}

int main() {

    int u,v,cap;
    std::memset(head,-1,sizeof(head));

    scanf("%d%d%d%d",&N,&M,&S,&T);
    for(int i=1;i<=M;++i) {
        scanf("%d%d%d",&u,&v,&cap);
        add(u,v,cap); add(v,u,0);
    }

    dinic();
    printf("%d",ans);
    return 0;
}