Given two non-negative integers num1 and num2 represented as strings, return the product of num1 and num2, also represented as a string.
Note:
- The length of both num1 and num2 is < 110.
- Both num1 and num2 contain only digits 0-9.
- Both num1 and num2 do not contain any leading zero, except the number 0 itself.
- You must not use any built-in BigInteger library or convert the inputs to integer directly.
Example 1: Input: num1 = "2", num2 = "3" Output: "6" Example 2: Input: num1 = "123", num2 = "456" Output: "56088"