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:

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.

#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;
}