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 

Credits:

Special thanks to @ifanchu for adding this problem and creating all test cases.

public class Solution {
    public String convertToTitle(int n) {
        char a[]=new char[27];

        a[0]='A';
        for(int i=1;i<=26;i++){
            a[i]=(char)(a[0]+i-1);
        }
        if(n<=26)
            return (a[n]+"");
        String s="";
        while(n>26){
            int r = n%26;
            if(r==0){
                r=26;
                n-=26;
            }
            s = a[r]+s;
            n/=26;
        }
        s = a[n]+s;
        return s;
    }
}