zoukankan      html  css  js  c++  java
  • POJ 1273:Drainage Ditches(EK 最大流)

    http://poj.org/problem?id=1273

    Description

    Every time it rains on Farmer John's fields, a pond forms over Bessie's favorite clover patch. This means that the clover is covered by water for awhile and takes quite a long time to regrow. Thus, Farmer John has built a set of drainage ditches so that Bessie's clover patch is never covered in water. Instead, the water is drained to a nearby stream. Being an ace engineer, Farmer John has also installed regulators at the beginning of each ditch, so he can control at what rate water flows into that ditch.
    Farmer John knows not only how many gallons of water each ditch can transport per minute but also the exact layout of the ditches, which feed out of the pond and into each other and stream in a potentially complex network.
    Given all this information, determine the maximum rate at which water can be transported out of the pond and into the stream. For any given ditch, water flows in only one direction, but there might be a way that water can flow in a circle.

    Input

    The input includes several cases. For each case, the first line contains two space-separated integers, N (0 <= N <= 200) and M (2 <= M <= 200). N is the number of ditches that Farmer John has dug. M is the number of intersections points for those ditches. Intersection 1 is the pond. Intersection point M is the stream. Each of the following N lines contains three integers, Si, Ei, and Ci. Si and Ei (1 <= Si, Ei <= M) designate the intersections between which this ditch flows. Water will flow through this ditch from Si to Ei. Ci (0 <= Ci <= 10,000,000) is the maximum rate at which water will flow through the ditch.

    Output

    For each case, output a single integer, the maximum rate at which water may emptied from the pond.

    Sample Input

    5 4
    1 2 40
    1 4 20
    2 4 20
    2 3 30
    3 4 10
    

    Sample Output

    50

    题意分析:

    n个点m条边,求1到n的最大流。

    #include <stdio.h>
    #include <algorithm>
    #include <string.h>
    #include <queue>
    using namespace std;
    #define N 220
    int n, m;
    int e[N][N], pre[N], flow[N], inf=0x3f3f3f; 
    queue<int>q;
    int bfs(int s, int t)
    {
    	int u, v;
    	while(!q.empty())
    		q.pop();
    	memset(pre, -1, sizeof(pre));
    	q.push(s);pre[s]=0;flow[s]=inf;
    	while(!q.empty())
    	{
    		u=q.front();
    		q.pop();
    		if(u==t)
    			break;
    		for(v=1; v<=n; v++)
    		{
    			if(v!=s && e[u][v] && pre[v]==-1)
    			{
    				q.push(v);
    				pre[v]=u;
    				flow[v]=min(flow[u], e[u][v]);
    			}
    		}
    	}
    	if(pre[t]==-1)
    		return -1;
    	return flow[t];
    }
    int EK(int s, int t)
    {
    	int ans=0, d, p;
    	while(1)
    	{
    		d=bfs(s, t);
    		if(d==-1)
    			break;
    		p=t;
    		while(p!=s)
    		{
    			e[pre[p]][p]-=d;
    			e[p][pre[p]]+=d;
    			p=pre[p];
    		}
    		ans+=d;	
    	}
    	
    	return ans;
    }
    
    int main()
    {
    	int u, v, w, i;
    	while(scanf("%d%d", &m, &n)!=EOF)
    	{
    		memset(e, 0, sizeof(e));
    		memset(flow, 0, sizeof(flow));
    		for(i=0; i<m; i++)
    		{
    			scanf("%d%d%d", &u, &v, &w);
    			e[u][v]+=w;
    		}
    		printf("%d
    ", EK(1, n));	
    	}
    	return 0;
    }
  • 相关阅读:
    ReactNative 分享解决listView的一个郁闷BUG
    SDWebImage 图片下载缓存框架 常用方法及原理
    巧谈GCD
    Core Bluetooth下实现两个设备进行互联
    iOS开发Delegate,Notification,Block使用心得
    iOS开发之性能优化
    iOS开发之git学习
    iOS开发之自定义输入框(利用UITextField及UITextView)
    iOS开发之网络请求(基于AFNetworking的再封装)
    iOS开发之设置界面的抽取
  • 原文地址:https://www.cnblogs.com/zyq1758043090/p/11852590.html
Copyright © 2011-2022 走看看