Set

Intuition Most of the cases, if the question asks for O(1), there’s probably a set or hashmap involced. This question uses the set to check if the next number is in the set (making a sequence) Approach The key is that the set provides a near O(1) access, making the solution O(n) Complexity Time complexity: Space complexity: Code class Solution(object): def longestConsecutive(self, nums): """ :type nums: List[int] :rtype: int """ longest = 0 num_set = set(nums) for num in num_set: if num-1 not in num_set: current = num curr_streak = 1 while current + 1 in num_set: current += 1 curr_streak += 1 longest = max(curr_streak, longest) return longest