LeetCode in Racket

128. Longest Consecutive Sequence

Medium

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

Constraints:

• 0 <= nums.length <= 105
• -109 <= nums[i] <= 109

Solution

(define (longest-consecutive nums)
  (define uniq (list->set nums))

  (define (seq x a)
    (cond
      [(set-member? uniq x) (seq (add1 x) (add1 a))]
      [else a]))

  (let loop ([l (set->list uniq)] [m 0])
    (match l
      ['() m]
      [(cons x xs) #:when (not (set-member? uniq (sub1 x))) (loop xs (max m (seq x 0)))]
      [(cons x xs) (loop xs m)])))

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