Through Brute Force:> cpp solution

string isValid(string s) { int freq[26]={0}; for (char c:s) { freq[c-'a']++; }

sort(freq,freq+26);

if ((freq[24]==0) && (freq[25]!=0))
{
    return "YES";
}
int j=0;
while(freq[j]==0)
{
    j++;
}
if (freq[j]==freq[25])
{
    return "YES";
}

else if ((freq[j]==1) &&  freq[j+1]==freq[25])
{
    return "YES";
}

else if((freq[25]-freq[j]==1) && freq[j]==freq[24])
{
    return "YES";
}

else
{
    return "NO";
}

}