关于深度优先遍历（DFS）
概念：深度优先搜索是搜索的手段之一，它从某个状态开始，不断地转移状态直到无法移动，然后再回退到前一步的状态，继续转移到其他状态，如此不断重复直到找到最终的解。（一条路走到黑不到黄河不死心，碰壁才回头。）
访问顺序如上图所示
例题如下：
部分和问题
**
描述
给定整数a1、a2、…an，判断是否可以从中选出若干数，使它们的和恰好为K。
输入
首先，n和k，n表示数的个数，k表示数的和。
接着一行n个数。（1<=n<=20,保证不超int范围）**
输出
如果和恰好可以为k，输出“YES”，并按输入顺序依次输出是由哪几个数的和组成，否则“NO”
解题思路：从a1开始，看之后的每个数加或不加，在全部数都决定后再判断和是否等于k即可。
代码如下：
#include运行结果如下：
还可以将最终结果记忆化：
#include运行结果如下：
例题：POJ.2386:
Lake Counting
Time Limit: 1000MS Memory Limit: 65536K
Total Submissions: 53321 Accepted: 26073
Description
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.
Input
- 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.
Output - Line 1: The number of ponds in Farmer John’s field.
Sample Input
10 12
W…WW.
.WWW…WWW
…WW…WW.
…WW.
…W…
…W…W…
.W.W…WW.
W.W.W…W.
.W.W…W.
…W…W.
Sample Output
3
Hint
OUTPUT DETAILS:
There are three ponds: one in the upper left, one in the lower left,and one along the right side.
题目思路：
本题是要你找出水洼个数W，并且W的上，下，左，右，及左上，左下，右上，右下，这八个部分表示同一个水洼，我们只需用深度优先搜索，搜索它的这些部分的所有W,然后搜索次数则为遍历次数，每搜索到一个W时则让这个W变为’ . '。
AC代码如下：
#include再看一道中等题：
【CodeForces】510B - Fox And Two Dots（bfs）
Fox Ciel is playing a mobile puzzle game called “Two Dots”. The basic levels are played on a board of size n × m cells, like this:
Each cell contains a dot that has some color. We will use different uppercase Latin characters to express different colors.
The key of this game is to find a cycle that contain dots of same color. Consider 4 blue dots on the picture forming a circle as an example. Formally, we call a sequence of dots d1, d2, …, dk a cycle if and only if it meets the following condition:
These k dots are different: if i ≠ j then di is different from dj.
1.k is at least 4.
2.All dots belong to the same color.
3.For all 1 ≤ i ≤ k - 1: di and di + 1 are adjacent. Also, dk and d1 should also be adjacent. Cells x and y are called adjacent if they share an edge.
Determine if there exists a cycle on the field.
Examples
Input
3 4
AAAA
ABCA
AAAA
Output
Yes
Input
3 4
AAAA
ABCA
AADA
Output
No
Input
4 4
YYYR
BYBY
BBBY
BBBY
Output
Yes
Input
7 6
AAAAAB
ABBBAB
ABAAAB
ABABBB
ABAAAB
ABBBAB
AAAAAB
Output
Yes
Input
2 13
ABCDEFGHIJKLM
NOPQRSTUVWXYZ
Output
No
Note
In first sample test all ‘A’ form a cycle.
In second sample there is no such cycle.
The third sample is displayed on the picture above (‘Y’ = Yellow, ‘B’ = Blue, ‘R’ = Red).
本题思路：就是找出相同字母所能围成的圈且相同字母的个数为四。采用深搜，找到相同的则继续向下遍历一直到最后，看最后的那个字母是否与初始的相同，如果相同则输出YES,反之则继续遍历，最终没有找到则输出NO。
AC代码如下：
#include运行结果如下：
