CodeForces 510B Fox And Two Dots（DFS）

题意：给你一个n*m的网格，然后问你是否有只含有一种元素的环

思路：记录一下DFS即可

#include
using namespace std;
int n,m,flag;
char mp[55][55];
int vis[55][55];
int dir[4][2]={{0,1},{0,-1},{1,0},{-1,0}};
void dfs(int x,int y,char color,int fx,int fy)
{
	vis[x][y]=1;
	if (flag)
		return;
	for (int i = 0;i<4;i++)
	{
         int dx = x+dir[i][0];
		 int dy = y+dir[i][1];
		 if (dx<0 || dy<0 || dx>=n || dy>=m)
			 continue;
		 if (dx==fx && dy==fy)
			 continue;
		 if (mp[dx][dy]!=color)
			 continue;
		 if (vis[dx][dy])
		 {
			 flag=1;
			 return;
		 }
		 dfs(dx,dy,color,x,y);
	}
}
int main()
{
    scanf("%d%d",&n,&m);
	flag=0;
	for (int i = 0;i

Description

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

1. These k dots are different: if i ≠ j then d_i is different from d_j.
2. k is at least 4.
3. All dots belong to the same color.
4. 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.

Sample Input

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

Hint

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).