POJ 1293 网络流 第一题

完全的模板，做多了就好了吧

反向流量真的很有意思，有这样一种说法比较容易理解。”正向是+，反向就是-，其实是等价的。因为每次找到的增广路不一定是最优解里面的，所以再进行后面的操作的时候要重新选择，而反向流量就类似于回溯的作用。相当于撤销了刚才选择的u->v的流“。

 #include <iostream>
 #include <cstdio>
 #include <queue>
 #include <algorithm>
 #define INF 999999999
 //#define OPEN_FILE
 using namespace std;

 const int MAXN = , MAXM = , MAXL = ;
 struct Node
 {
     int id, v;
 };
 int cap[MAXM][MAXM], flow[MAXM][MAXM], a[MAXM], f[MAXM];
 int ans, n, m;
 void ek_bfs()
 {
     int u, v, s, t;
     s = ;
     t = m;
     ans = ;
     queue<int> d;
     while (){
         memset(a, , sizeof(a));
         a[s] = INF;
         d.push(s);
         while (!d.empty()){
             u = d.front();
             d.pop();
             for (v = s; v <= t; v++){
                 if (a[v] || cap[u][v] <= flow[u][v]) continue;
                 f[v] = u;
                 d.push(v);
                 a[v] = min(a[u], cap[u][v] - flow[u][v]);
             }
         }
         if (a[t] == ) break;
         for (u = t; u != s; u = f[u]){
             flow[f[u]][u] += a[t];
             flow[u][f[u]] -= a[t];
         }
         ans += a[t];
     }
 }
 int main()
 {
 #ifdef OPEN_FILE
     freopen("in.txt", "r", stdin);
     freopen("out.txt", "w", stdout);
 #endif // OPEN_FILE
     int i, x, y, z;
     while (~scanf("%d%d", &n, &m)){
         memset(flow, , sizeof(flow));
         memset(cap, , sizeof(cap));
         for (i = ; i <= n; i++){
             scanf("%d%d%d", &x, &y, &z);
             cap[x][y] += z;
         }
         ek_bfs();
         printf("%d\n", ans);
     }
 }