【题解】CODEVS 1993 草地排水

#include<cstdio>
#include<climits>
#include<cstring>
#include<algorithm>

const int MAXN=20010;
const int MAXM=500010;
using namespace std;

struct edge
{
	int v,next,val;
}e[MAXM];
int p[MAXN],eid;
int n,m;
int h[MAXN];
int gap[MAXN];
int S,T;

inline void add(int from,int to,int val)
{
    e[eid].v=to;e[eid].val=val;
    e[eid].next=p[from];
    p[from]=eid++;
}

inline int dfs(int pos,int cost)
{
    if(pos==T)
		return cost;
    int j,minh=n-1,lv=cost,d;
    for(j=p[pos];j!=-1;j=e[j].next)
    {
        int v=e[j].v,val=e[j].val;
        if(val>0)
        {
            if(h[v]+1==h[pos])
            {
                if(lv<e[j].val) 
					d=lv;
                else 
					d=e[j].val;
                d=dfs(v,d);
                e[j].val-=d;
                e[j^1].val+=d;
                lv-=d;
                if(h[S]>=n) 
					return cost-lv;
                if(lv==0) 
					break;
            }
            if (h[v]<minh)
				minh=h[v];
        }
    }
    if (lv==cost)
    {
        --gap[h[pos]];
        if(gap[h[pos]]==0)
			h[S]=n;
        h[pos]=minh+1;
        ++gap[h[pos]];
    }
    return cost-lv;
}

int isap(int st,int ed)
{
    S=st;
	T=ed;
    int ret=0;
    memset(gap,0,sizeof(gap));
    memset(h,0,sizeof(h));
    gap[st]=n;
    while(h[st]<n)
        ret+=dfs(st,INT_MAX);
    return ret;
}
int main()
{
    scanf("%d%d",&m,&n);
    memset(p,-1,sizeof(p));
    eid=0;
    for(int i=1;i<=m;++i)
    {
        int u,v,c;
        scanf("%d%d%d",&u,&v,&c);
        add(u,v,c);
        add(v,u,0);
    }
    printf("%d\n",isap(1,n));
    return 0;
}