C :

#include<stdio.h>
int main()
{
    int n,m,i,j,t;
    char  a[101][101];
    while(~scanf("%d%d\n",&n,&m))
    {t=0;
        for(i=0;i<n;i++)
            gets(a[i]);
            for(i=0;i<n-1;i++)
                if(a[i][0]==a[i+1][0])  {t=1;printf("NO\n");break;}
            if(t==0)
            {t=0;
                for(i=0;i<n;i++)
                  {for(j=0;j<m-1;j++)
                      if(a[i][j]!=a[i][j+1])  t++;
                    if(t!=0)  break;
                  }
                if(t!=0)  printf("NO\n");
                else      printf("YES\n");
            }
    }
    return 0;
}

C++ :

#include<iostream>
#include<cstdio>
#include<string.h>
#include<algorithm>
#include<math.h>
#include<cstring>
using namespace std;

int n, m;
char pre, map[110][110];
int main()
{
    while(scanf("%d%d", &n, &m) != EOF)
    {
        int flag = 1;
        memset(map, 0, sizeof(map));
        for(int i = 1; i <= n; i++)
            cin >> map[i];
        pre = 'a';
        for(int i = 1; i <= n; i++)
        {
            if(map[i][0] == pre)
            {
                flag = 0;
                break;
            }
            else
            {
                for(int j = 1; j < m; j++)
                {
                    if(map[i][j] != map[i][0])
                    {
                        flag = 0;
                        break;
                    }
                }
            }
            if(flag == 0)
                break;
            pre = map[i][0];
        }
        if(flag == 1)
            printf("YES\n");
        else
            printf("NO\n");
    }
    return 0;
}