Given an unsorted list of integers, return the length of the longest sequence of consecutive integers. The integers in the sequence do not need to be adjacent in the list, only consecutive in value. The expected complexity is O(n). Return 0 for null or empty input. Duplicates should not extend a sequence.
Examples:
Input: nums = [100, 4, 200, 1, 3, 2]
Output: 4
Explanation: The longest consecutive sequence is [1, 2, 3, 4]
Input: nums = [9, 1, 4, 7, 3, 8, 6, 2, 5]
Output: 9
Explanation: All values from 1 to 9 form a consecutive sequence
Input: nums = [1, 2, 0, 1]
Output: 3
Explanation: The duplicate 1 is ignored. The longest sequence is [0, 1, 2]
Input: nums = [5]
Output: 1
Explanation: A single value is its own sequence of length one
Input: nums = null
Output: 0
Explanation: Null input returns zero
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