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 

string convertToTitle(int n) {
        string str;
        while(n){
            int r=n%26;
            n=n/26;
            if(r==0){   //为26的整数倍，该位设置为Z，n减掉1
                str+='Z';
                n--;
            }else{
                str+=('A'+r-1);
            }
        }
        //反转
        string result;
        for(int i=str.size()-1;i>=0;i--){
            result+=str[i];
        }
        return result;
    }