Longest Consecutive Sequence [LC#128]

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

Brute Force

• Consider each element as seed.
• Check if next element in the sequece is in the list. keep track of sequence length
• $T(n) = O(n^3)$; $S(n) = O(1)$

Sorting

• Sort the array.
• Check the length of longest conseqtive sequence
• $T(n) = O(n \log n)$; $S(n) = O(n)$

Hash Table based look up

• For each num, if num-1 is not in the list, then its possibly a sequence’s begning
• For each sequence begning, check if conseqtive elements are in the list
• Each valid sequence is tested once
• $T(n) = O(n)$
• $S(n) = O(n)$

def longest_consecutive_sequence(nums: List[int]) -> int:
    entries = set(nums)
    best = 0
    for num in nums:
        if num-1 not in entries:
            next_num, seq_len = num+1, 1
            while next_num in entries: 
                next_num += 1
                seq_len += 1
            best = max(best, seq_len)
    return best