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.
0 <= nums.length <= 10^5, -10^9 <= nums[i] <= 10^9
Use a HashSet for O(1) lookup. For each number, check if it is the start of a sequence (num-1 not in set), then count consecutive numbers.
def longestConsecutive(nums):
num_set = set(nums)
longest = 0
for num in num_set:
if num - 1 not in num_set:
current = num
streak = 1
while current + 1 in num_set:
current += 1
streak += 1
longest = max(longest, streak)
return longest