【最大流】北大 poj 1273 Drainage Ditches

/* THE PROGRAM IS MADE BY PYY */ /*----------------------------------------------------------------------------// Copyright (c) 2011 panyanyany All rights reserved. URL : http://poj.org/problem?id=1273 Name : 1273 Drainage Ditches Date : Sunday, January 1, 2012 Time Stage : seven to eight hours around Result: 9696192 panyanyany 1273 Accepted 348K 16MS C++ 1974B 2012-01-01 15:55:41 9696127 panyanyany 1273 Wrong Answer C++ 2316B 2012-01-01 15:03:34 9696116 panyanyany 1273 Wrong Answer C++ 2319B 2012-01-01 14:55:28 Test Data : Review : 最大流，看了许久，大部分都可以慢慢看懂，唯一难搞的就是增加反向边的问题， 看到大牛的例子，才明白反向边确实很重要。 一开始看的是这位大牛的文章： http://www.cppblog.com/mythit/archive/2009/04/19/80470.aspx 这篇文章则给出了例子，证明反向边确实重要： http://pictureyong.blog.163.com/blog/static/851870682011299044852/ 这位大牛的解释挺详细的： http://blog.youkuaiyun.com/akof1314/article/details/4845183 听华神说这种方法是最慢的，当时有脑没脑地想了一下，跟深搜比起来，广搜应该是很快的了。 后来突然意识到，这种算法不止搜了一次！也就是说，如果一张图很大的话，这种算法可能会 需要对其反复搜索，而每次大范围的搜索却又只找到一条路，如果路不对，还要继续找……于是 这样就会很费时间了……呃，好吧，我也是刚学，进阶的事慢慢来…… //----------------------------------------------------------------------------*/ #include <stdio.h> #include <string.h> #include <queue> using namespace std ; #define DEBUG #define min(x, y) ((x) < (y) ? (x) : (y)) #define INF 0x3f3f3f3f #define MAXN 202 int n, m ; int map[MAXN][MAXN], path[MAXN], flow[MAXN] ; int Mark_Point (const int start, const int end) { int t, i, j ; queue<int> q ; memset (path, 0, sizeof (path)) ; flow[start] = INF ; q.push (start) ; // puts ("---------------------Loop:") ; while (!q.empty ()) { t = q.front () ; q.pop () ; if (end == t) break ; // printf ("Point: %d\n", t) ; for (i = 1 ; i <= end ; ++i) { if ((i != start) && map[t][i] && !path[i]) { path[i] = t ; flow[i] = min (flow[t], map[t][i]) ; // printf ("\tPoint: %d, (%d, %d)\n", i, path[i], flow[i]) ; q.push (i) ; } } } if (path[end] == 0) return -1 ; return flow[end] ; } int Edmonds_Karp (const int start, const int end) { int incr, step, curr, prev ; incr = 0 ; while ((step = Mark_Point (start, end)) != -1) { incr += step ; curr = end ; while (curr != start) { prev = path[curr] ; map[prev][curr] -= step ; map[curr][prev] += step ; // printf ("Point: %d --> Point: %d === %d\n", prev, curr, // map[prev][curr]) ; curr = prev ; } } return incr ; } int main (void) { int i, j ; int x, y, c ; while (~scanf ("%d%d", &n, &m)) { memset (map, 0, sizeof (map)) ; for (i = 0 ; i < n ; ++i) { scanf ("%d%d%d", &x, &y, &c) ; map[x][y] += c ; } printf ("%d\n", Edmonds_Karp (1, m)) ; } return 0 ; }