Given two non$-$negative integers num$1$ and num$2$ represented as strings, return the product of num$1$ and num$2,$ also represented as a string.

Note:$$You must not use any built$-$in BigInteger library or convert the inputs to integer directly.

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Example $1$:

Input: num$1="2",$ num$2="3"$ Output: $"6"$

Example $2$:

Input: num$1="123",$ num$2="456"$ Output: $"56088"$

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Constraints:

$1<=$ num$1.$length, num$2.$length $<=200$

num$1$ and num$2$ consist of digits only.

Both num$1$ and num$2$do not contain any leading zero, except the number $0$itself.