Given an integer `columnNumber`, return its corresponding column title as it appears in an Excel sheet.

For example:

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

Example 1:

    Input: columnNumber = 1
    Output: "A"

Example 2:

    Input: columnNumber = 28
    Output: "AB"

Example 3:

    Input: columnNumber = 701
    Output: "ZY"

Constraints:

- `1 <= columnNumber <= 231 - 1`