TikTok OA 面试真题解析：Longest Consecutive Sequence（最长连续序列）

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.

Example 1:

Input: nums = [100,4,200,1,3,2]

Output: 4

Explanation: The longest consecutive elements sequence is [1,2,3,4]. Therefore its length is 4.

Example 2:

Input: nums = [0,3,7,2,5,8,4,6,0,1]

Output: 9

这题的核心是用哈希集合把查找连续性降到 O(1)，然后只从“序列起点”开始扩展：当某个数的前一个数不存在时，说明它可能是一个连续段的开头，再向后统计长度即可。这样每个元素最多被处理一次，总时间复杂度可以做到 O(n)，也满足题目要求。示例中 [100,4,200,1,3,2] 的最长连续序列是 [1,2,3,4]，长度为 4；[0,3,7,2,5,8,4,6,0,1] 则能连成 0 到 8 的完整序列，答案为 9。