Longest Consecutive Sequence

Problem

Given an unsorted integer array, return the length of the longest sequence of consecutive integers (e.g. 4,5,6,7). Must run in O(n) — sorting is O(n log n) and won't count. Example: [100,4,200,1,3,2] → 4 because [1,2,3,4] is the longest consecutive run.

Constraints

• ›0 ≤ nums.length ≤ 10⁵
• ›-10⁹ ≤ nums[i] ≤ 10⁹
• ›O(n) time required