168\. Excel Sheet Column Title

Easy

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" 

**Example 4:**

**Input:** columnNumber = 2147483647

**Output:** "FXSHRXW" 

**Constraints:**

*   <code>1 <= columnNumber <= 2<sup>31</sup> - 1</code>