1 #include <bits/stdc++.h> 2 using namespace std; 3 const int maxn = 2505; 4 int n, m; 5 int fa[maxn]; 6 char s[55][55]; 7 //int dx[]={1, 1, 0, 0} 8 int ety(int x, int y) { 9     return x*m+y;10 }11 void init(int n) {12     for(int i=0; i<=n; i++) {13         fa[i] = i;14     }15 }16 int get(int x) {17     return fa[x] == x ? x : fa[x] = get(fa[x]);18 }19 bool join(int x, int y) {20     int rx = get(x), ry = get(y);21     if(rx == ry) return false;22     fa[rx] = fa[ry] = min(rx, ry);23     return true;24 }25 int main() {26 27     while(~scanf("%d%d", &n, &m)) {28         getchar();29         for(int i=0; i<n; i++) {30             gets(s[i]);31         }32         init(n*m);33         bool ok = true;34         for(int i=0; i<n; i++) {35             for(int j=0; j<m; j++) {36                 if(i!=n-1 && s[i][j] == s[i+1][j]) ok = join(ety(i, j), ety(i+1, j));37                 if(!ok) break;38                 if(j!=m-1 && s[i][j] == s[i][j+1]) ok = join(ety(i, j), ety(i, j+1));39                 if(!ok) break;40             }41             if(!ok) break;42         }43         if(ok) printf("No\n");44         else printf("Yes\n");45     }46     return 0;47 }