hdu 1532 Network Flow Drainage Ditches

最新推荐文章于 2020-08-15 12:58:23 发布

原创最新推荐文章于 2020-08-15 12:58:23 发布 · 422 阅读

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#acm #hdu #Network Flow #ISAP

本文介绍了一个使用ISAP算法解决最大流问题的C++实现案例。这是作者首次尝试解决网络流问题，通过一个具体题目（HDU 1532）展示了如何运用模板代码来快速求解。尽管作者并未深入探讨ISAP算法的具体细节，但该实现为理解最大流问题提供了一个实用的起点。

Using the code solve max_flow problem.I just using the mould.I don't know the details of isap algorithm.

ps:This is my first solve Network Flow problem.

The portal : http://acm.hdu.edu.cn/showproblem.php?pid=1532

#include<iostream>
#include <cstdio>
#include <cmath>
#include <memory.h>

#define MAXN 100005
#define MAXE 2000000
#define INF 0x3f3f3f3f

using namespace std;

int tmp,src,des,cnt;
int n,m;

struct Edge{
	int from,to;
	int next,cap;
}edge[MAXE];

int head[MAXN];
int gap[MAXN],dep[MAXN],cur[MAXN],stack[MAXN],top;

int isap(){
	int cur_flow,max_flow,u,insert,i;
	memset(dep,0,sizeof(dep));
	memset(gap,0,sizeof(gap));
	memcpy(cur,head,n);
	max_flow = 0;
	u = src;
	top = 0;
	while(dep[src] < n){
		if(u == des){
			cur_flow = INF;
			for(i = 0;i < top;i++){
				if(cur_flow > edge[stack[i]].cap){
					insert = i;
					cur_flow = edge[stack[i]].cap;
				}
			}
			for(i = 0;i < top;i++){
				edge[stack[i]].cap -= cur_flow;
				edge[stack[i]^1].cap += cur_flow;
			}
			max_flow += cur_flow;
			u = edge[stack[top = insert]].from;
		}
		for(i = cur[u];i != -1;i=edge[i].next){
			if((edge[i].cap > 0)&&(dep[u] == dep[edge[i].to]+1))
				break;
		}
		if(i != -1){
			stack[top++] = i;
			u = edge[i].to;
		}
		else{
			if(0 == --gap[dep[u]])break;
			int minn = n;
			for(i = head[u];i!=-1;i=edge[i].next){
				if(edge[i].cap > 0)
					minn = (minn > dep[edge[i].to]) ? (cur[u] = i,dep[edge[i].to]) : minn;
			}
			++gap[dep[u] = minn + 1];
			if(u != src) u = edge[stack[--top]].from;
		}
	}
	return max_flow;
}

void addedge(int u,int v,int f){
	edge[cnt].next = head[u];
	edge[cnt].from = u;
	edge[cnt].to = v;
	edge[cnt].cap = f;
	head[u] = cnt ++;
	edge[cnt].next = head[v];
	edge[cnt].from = v;
	edge[cnt].to = u;
	edge[cnt].cap = 0;
	head[v] = cnt ++;
}

int main(void){
	while(~scanf("%d%d",&m,&n)){
		cnt = 0;
		src = 0;
		des = n-1;
		memset(head,-1,sizeof(head));
		while(m--){
			int a,b,c;
			scanf("%d %d %d",&a,&b,&c);
			--a; --b;
			addedge(a,b,c);
		}
		int ans = isap();
		printf("%d\n",ans);
	}
	return 0;
}