Leetcode•Apr 27, 2025

Longest Consecutive Sequence

Hazrat Ali

Leetcode

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

Example 3:

Input: nums = [1,0,1,2]
Output: 3

Solution
/**
 * @param {number[]} nums
 * @return {number}
 */
const longestConsecutive = nums => {
  let max = 0;

  const set = new Set();

  for (let i = 0; i < nums.length; i++) {
    set.add(nums[i]);
  }

  for (let i = 0; i < nums.length; i++) {
    let count = 1;

   
    let num = nums[i];
    while (set.has(--num)) {
      count++;
      set.delete(num);
    }

    num = nums[i];
    while (set.has(++num)) {
      count++;
      set.delete(num);
    }

    max = Math.max(max, count);
  }

  return max;
};



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