Only trick is the 'Z'. Because when the number % 26 == 0, means there is a 'Z'. But when you n/=26, there is an extra 1 added to the n. EX:

27 % 26 = 1, 27 / 26 = 1  == > AA

26 % 26 = 0, 26 / 26 = 1  Here this extra one need to be removed. You can imagine that 'Z' becomes next around of the loop.

 

1 class Solution { 2 public: 3     string convertToTitle(int n) { 4         string result; 5         while (n > 0) { 6             int tmp = n%26; 7             n /= 26; 8             if (tmp == 0) { 9                 result = 'Z' + result;10                 n--;11             } else {12                 result = char(tmp + 'A' - 1) +result;13             }14         }15         return result;16     }17 };