Longest Consecutive Sequence

Longest Consecutive Sequence

Given an unsorted array of integers nums, return the length of the longest consecutive elements sequence.

You must write an algorithm that runs in O(n) time.

Examples

Example 1:

Input: nums = [100, 4, 200, 1, 3, 2]
Output: 4
Explanation: The longest consecutive sequence is [1, 2, 3, 4]. Its length is 4.

Example 2:

Input: nums = [0, 3, 7, 2, 5, 8, 4, 6, 0, 1]
Output: 9
Explanation: The longest consecutive sequence is [0, 1, 2, 3, 4, 5, 6, 7, 8]. Its length is 9.

Example 3:

Input: nums = []
Output: 0

Constraints

• 0 <= nums.length <= 10^5
• -10^9 <= nums[i] <= 10^9

Expected Complexity

• Time: O(n)
• Space: O(n)