Given two non-negative integers num1 and num2 represented as strings, return their product as a string.
Do not convert the inputs directly to built-in big integers.
Examples
Input: num1 = "2", num2 = "3" Output: "6"
Input: num1 = "123", num2 = "456" Output: "56088"
Constraints
- 1 <= num1.length, num2.length <= 200
- num1 and num2 consist of digits only.