計蒜課/排澇(Edmond-Karp)

Posted 2020-09-16 ygeloutingyu

題目鏈接: https://nanti.jisuanke.com/t/36

 

題意:中文題目誒~

 

思路: 最大流模板題....

關於最大流算法blog:

　　http://www.cnblogs.com/zsboy/archive/2013/01/27/2878810.html

　　http://blog.csdn.net/y990041769/article/details/21026445

　　http://www.cnblogs.com/luweiseu/archive/2012/07/14/2591573.html

 

代碼:

 1 #include <iostream>
 2 #include <stdio.h>
 3 #include <queue>
 4 #include <string.h>
 5 using namespace std;
 6 
 7 const int MAXN=2e2+10;
 8 const int inf=0x7fffffff;
 9 int capacity[MAXN][MAXN];//記錄殘留網絡流量
10 int flow[MAXN]; //標記從源點到當前節點實際還剩多少流量可用
11 int pre[MAXN]; //pre[i]爲i的父親節點
12 int n, m;
13 
14 int bfs(int src, int des){
15     queue<int> q;
16     while(!q.empty()){
17         q.pop();
18     }
19     for(int i=1; i<=m; i++){
20         pre[i]=-1;
21     }
22     pre[src]=0;
23     flow[src]=inf;
24     q.push(src);
25     while(!q.empty()){
26         int indx=q.front();
27         q.pop();
28         if(indx==des) break;//找到了增廣徑
29         for(int i=1; i<=m; i++){
30             if(i!=src&&capacity[indx][i]>0&&pre[i]==-1){
31                 pre[i]=indx;
32                 flow[i]=min(capacity[indx][i], flow[indx]);
33                 q.push(i);
34             }
35         }
36     }
37     if(pre[des]==-1) return -1;
38     return flow[des];
39 }
40 
41 int edmond_karp(int src, int des){
42     int increasement=0;
43     int sumflow=0;
44     while((increasement=bfs(src, des))!=-1){
45         int k=des; //利用前區尋找路徑
46         while(k!=src){
47             int last=pre[k];
48             capacity[last][k]-=increasement;//改變正向邊容量
49             capacity[k][last]+=increasement;//改變反向邊容量
50             k=last;
51         }
52         sumflow+=increasement;
53     }
54     return sumflow;
55 }
56 
57 int main(void){
58     int start, end, ci;
59     while(~scanf("%d%d", &n, &m)){
60         memset(capacity, 0, sizeof(capacity));
61         memset(flow, 0, sizeof(flow));
62         for(int i=0; i<n; i++){
63             scanf("%d%d%d", &start, &end, &ci);
64             if(start==end) continue;
65             capacity[start][end]+=ci;//可能有重邊
66         }
67         printf("%d\\n", edmond_karp(1, m));
68     }
69     return 0;
70 }

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