USACO 4.2.1 网络流 SAP

/*
ID: shenxyy1
PROG: ditch
LANG: C++
*/
#include<stdio.h>
#include<stdlib.h>
#include<string.h>
#include<iostream>
#include<algorithm>
#define For(i,j,k) for (i=j;i<=k;i++)
#define max(x,y) ((x)>(y)?(x):(y))
#define min(x,y) ((x)<(y)?(x):(y))
using namespace std;

const int dmax=1010,INF=1000000000;
int a[dmax][dmax],d[dmax],q[dmax],gap[dmax],fa[dmax],low[dmax],m,n,ans;
bool p[dmax];

void bfs(){
	int f=0,l=1,i,k;
	memset(d,-1,sizeof(d));
	q[1]=n;
	d[n]=0;
	gap[0]++;
	while (f<l){
		k=q[++f];
		For(i,1,n)
			if (d[i]!=-1 && a[i][k]){
				d[i]=d[k]+1;
				gap[d[i]]++;
				q[++l]=k;
			}
	}
}
void sap(){
	int i,j,k,t,flag,min;
	bfs();
	t=1;
	memset(fa,-1,sizeof(fa));
	while (d[1]<n){
		flag=0;
		low[1]=INF;
		For(i,1,n)
			if (d[t]==d[i]+1 && a[t][i]){
				flag=1;
				break;
			}
		if (flag){
			low[i]=a[t][i];
			low[i]=min(low[t],low[i]);
			fa[i]=t;
			t=i;
			if (t==n){
				min=low[n];
				ans+=min;
				j=n;
				while (j!=1){
					a[fa[j]][j]-=min;
					a[j][fa[j]]+=min;
					j=fa[j];
				}
				t=1;
			}
		}else{
			min=n;
			For(i,1,n)
				if (a[t][i])
					min=min(d[i],min);
			gap[d[t]]--;
			if (gap[d[t]]==0)
				break;
			d[t]=min+1;
			gap[d[t]]++;
			if (t!=1)
				t=fa[t];
		}
	}
}

int main(){
	int i,j,k,x,y,z;
	freopen("ditch.in","r",stdin);
	freopen("ditch.out","w",stdout);
	scanf("%d%d",&m,&n);
	For(i,1,m){
		scanf("%d%d%d",&x,&y,&z);
		a[x][y]+=z;
	}
	sap();
	printf("%d\n",ans);
	return 0;
}