Longest Consecutive Sequence

Problem

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

/**
 * @param {number[]} nums
 * @return {number}
 */
var longestConsecutive = function(nums) {
    const map = {};
    for (const n of nums) {
        map[n] = true;
    }
    let max = 0;
    for (const n of nums) {
        if (!(n - 1 in map)) {
            let i = 1;
            while (n + i in map) {
                i++;
            }
            max = Math.max(i, max);
        }
    }
    return max;
};

Start off by storing all values in nums into a set. Next, iterate through nums and use the set to find the longest consecutive sequence (ie. by testing if values exist in the set). Notice that we only attempt to find the length of the sequence if the number being tested is the start of said sequence, this is needed for O(n) runtime.