Two non-negative integers are each represented as a singly linked list of digits (head = most significant digit). Multiply them and return the product as an integer.
Input: Two linked lists of single digits [0-9], head = most significant digit.
Output: Integer product of the two numbers.
Input: [1,2,3], [4,5,6]
Output: 56088
Explanation: 123 x 456 = 56088.Input: [9,9,9], [9,9,9]
Output: 998001
Explanation: 999 x 999 = 998001.Input: [0], [5]
Output: 0
Explanation: 0 x 5 = 0.1<=nodes<=9 (each)0<=Node.val<=9No leading zeros except single 0