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 

这道题要注意，这不是单纯的26进制，26进制应该是：

0 -> A
1 -> B
2 -> C
...
25 -> Z
26 -> AA
27 -> AB 

那我们处理的时候就干脆吧n直接减掉1

var convertToTitle = function(n) {
    var res="";
    while(n>0){
        res=String.fromCharCode(65+(n-1)%26)+res;
        n=parseInt((n-1)/26);
    }
    return res;
};