Openjudge1388 Lake Counting【DFS/Flood Fill】

http://blog.csdn.net/c20182030/article/details/52327948

1388:Lake Counting

总时间限制: 

 1000ms 

 内存限制: 

 65536kB

描述

Due to recent rains, water has pooled in various places in Farmer John's field, which is represented by a rectangle of N x M (1 <= N <= 100; 1 <= M <= 100) squares. Each square contains either water ('W') or dry land ('.'). Farmer John would like to figure out how many ponds have formed in his field. A pond is a connected set of squares with water in them, where a square is considered adjacent to all eight of its neighbors.

Given a diagram of Farmer John's field, determine how many ponds he has.

输入

* Line 1: Two space-separated integers: N and M

* Lines 2..N+1: M characters per line representing one row of Farmer John's field. Each character is either 'W' or '.'. The characters do not have spaces between them.

输出

* Line 1: The number of ponds in Farmer John's field.

样例输入

10 12
W........WW.
.WWW.....WWW
....WW...WW.
.........WW.
.........W..
..W......W..
.W.W.....WW.
W.W.W.....W.
.W.W......W.
..W.......W.

样例输出

3

提示

OUTPUT DETAILS:

There are three ponds: one in the upper left, one in the lower left,and one along the right side.

来源

USACO 2004 Novembe

题目大意  

大雨过后，农夫John的农场被水淹没不知所措有了积水，农场地图中‘W’代表积水，'.'代表干燥的土地。八连通的积水在同一个水洼里。农夫John想知道农场里有多少水洼。

这是一道搜索题。与其他搜索题一样，都是不断地搜索，直到达到目的。但是，这道题的目的是什么呢？

目的是求出有多少水洼，如果递归的话，怎么才能知道求出了数量呢？

所以这样，不能单纯的只调用一次函数（主程序中），否则最多只会找出一个水洼。

我一开始是这样想的，把每个水洼只留下一个W，其余换成干燥，最后再看有几个W。可是这样搜索的时候，留下的W也会被搜到，搜到就会变成'.'，答案就会是0。为了避免这个问题，需要设立一个辅助数组或者把这个W换成另一个字符，但是这样既浪费时间也浪费空间（还浪费我的脑容量）。

于是我把函数写在循环里，把一个水洼消灭干净后，水洼数量就加1。我用DFS，BFS实现了这个想法。