Given two lists of integers, return a new list created by alternating elements from each list. Start with the first element of list1, then first element of list2, then second of list1, and so on. If one list is longer, append the remaining elements at the end. Return an empty list if both inputs are null. If one is null, return the other.
Examples:
Input: list1 = [1, 3, 5], list2 = [2, 4, 6]
Output: [1, 2, 3, 4, 5, 6]
Explanation: Alternates between lists - 1 from list1, 2 from list2, 3 from list1, 4 from list2, 5 from list1, 6 from list2
Input: list1 = [1, 2], list2 = [3, 4, 5, 6]
Output: [1, 3, 2, 4, 5, 6]
Explanation: After exhausting list1, remaining elements from list2 are appended
Input: list1 = [10, 20, 30], list2 = [40]
Output: [10, 40, 20, 30]
Explanation: After list2 is exhausted, remaining elements from list1 are appended
Input: list1 = null, list2 = [1, 2]
Output: [1, 2]
Explanation: When one list is null, return the other list
Input: list1 = null, list2 = null
Output: []
Explanation: When both lists are null, return empty list
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