LeetCode：Excel Sheet Column Title

Given a positive integer, return its corresponding column title as appear in an Excel sheet.

For example:

1 -> A
2 -> B
3 -> C
...
26 -> Z
27 -> AA
28 -> AB 

class Solution {  
public:  
    string convertToTitle(int n) {  
        string ret, str;  
        if(n < 0) return ret;  

        while(n)  
        {  
            int tmp = n % 26;  
            n /= 26;  
            if(tmp == 0)  
            {  
                str += 'Z';  
                n--;  
            }  
            else  
            {  
                str += 'A' + tmp - 1;  
            }  
        }  

        for(int i = str.size()-1; i >= 0; i--)  
        {  
            ret += str[i];  
        }  

        return ret;  
    }  
};