ＣＦ510B Fox And Two Dots（搜索图形环）

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:

$\text{[math]}$

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, ..., d_k a cycle if and only if it meets the following condition:

These k dots are different: if i ≠ j then d_i is different from d_j.
k is at least 4.
All dots belong to the same color.
For all 1 ≤ i ≤ k - 1: d_i and d_i + 1 are adjacent. Also, d_k 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.

Input

The first line contains two integers n and m (2 ≤ n, m ≤ 50): the number of rows and columns of the board.

Then n lines follow, each line contains a string consisting of m characters, expressing colors of dots in each line. Each character is an uppercase Latin letter.

Output

Output "Yes" if there exists a cycle, and "No" otherwise.

Examples
input
Copy
3 4
AAAA
ABCA
AAAA

output
Copy
Yes

input
Copy
3 4
AAAA
ABCA
AADA

output
Copy
No

input
Copy
4 4
YYYR
BYBY
BBBY
BBBY

output
Copy
Yes

input
Copy
7 6
AAAAAB
ABBBAB
ABAAAB
ABABBB
ABAAAB
ABBBAB
AAAAAB

output
Copy
Yes

input
Copy
2 13
ABCDEFGHIJKLM
NOPQRSTUVWXYZ

output
Copy
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).

【题意】

给定n*m矩阵，看是否有相同的字符可以构成一个环

【分析】

爆搜~
注意：1、构成环至少需要4个字符
2、注意判断字符的来路

【代码】

　
#include<cstdio>
#include<cstdlib>
using namespace std;
const int N=105;
int n,m,ans,dir[4][2]={{0,1},{0,-1},{1,0},{-1,0}};
char mp[N][N];bool vis[N][N]={0};
void dfs(int x,int y,int px,int py,int step){
	vis[x][y]=1;
	for(int i=0;i<4;i++){
		int nx=x+dir[i][0];
		int ny=y+dir[i][1];
		if(nx<1||ny<1||nx>n||ny>m||mp[nx][ny]!=mp[x][y]) continue;
		if(!vis[nx][ny]) dfs(nx,ny,x,y,step+1);
		else{
			if((nx!=px||ny!=py)&&step>=4){puts("Yes");exit(0);}
		}
	}
}
inline void Init(){
	scanf("%d%d",&n,&m);
	for(int i=1;i<=n;i++) scanf("%s",mp[i]+1);
}
inline void Solve(){
	for(int i=1;i<=n;i++){
		for(int j=1;j<=m;j++){
			if(!vis[i][j]){
				dfs(i,j,0,0,1);
			}
		}
	}
	puts("No");
}
int main(){
	Init();
	Solve();
	return 0;
} 

__EOF__

本文作者：shenben
本文链接：https://www.cnblogs.com/shenben/p/10353229.html
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