poj1273 Drainage Ditches(最大流dinic板子)

Dinic算法的核心就在于多路增广。每次bfs给残余网络分层，然后一次dfs把能增广的全都增广了。复杂度是O(n2m).

#include <bits/stdc++.h>using namespace std;#define ll long long#define N 210#define inf 0x3f3f3f3finline int read(){    int x=0,f=1;char ch=getchar();    while(ch<'0'||ch>'9'){if(ch=='-')f=-1;ch=getchar();}    while(ch>='0'&&ch<='9') x=x*10+ch-'0',ch=getchar();    return x*f;}int n,m,h[N],num=1,lev[N],ans=0;struct edge{    int to,next,val;}data[N<<1];inline bool bfs(){    queue<int>q;memset(lev,0,sizeof(lev));    q.push(1);lev[1]=1;    while(!q.empty()){        int x=q.front();q.pop();        for(int i=h[x];i;i=data[i].next){            int y=data[i].to;if(lev[y]||!data[i].val) continue;            lev[y]=lev[x]+1;q.push(y);        }    }return lev[n];}int dinic(int x,int low){    if(x==n) return low;int tmp=low;//tmp--剩余流量     for(int i=h[x];i;i=data[i].next){        int y=data[i].to;if(lev[y]!=lev[x]+1||!data[i].val) continue;        int res=dinic(y,min(data[i].val,tmp));        if(!res) lev[y]=0;tmp-=res;data[i].val-=res;data[i^1].val+=res;        if(!tmp) return low;    }return low-tmp;}int main(){//  freopen("a.in","r",stdin);    m=read();n=read();    while(m--){        int x=read(),y=read(),val=read();        data[++num].to=y;data[num].next=h[x];h[x]=num;data[num].val=val;        data[++num].to=x;data[num].next=h[y];h[y]=num;data[num].val=0;    }while(bfs()) ans+=dinic(1,inf);    printf("%d\n",ans);    return 0;}