LeetCode 217. Contains Duplicate

The Problem

Link to original problem on Leetcode.

Given an integer array nums, return true if any value appears at least twice in the array, and return false if every element is distinct.


Example 1:

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

Example 2:

Input: nums = [1,2,3,4]
Output: false

Example 3:

Input: nums = [1,1,1,3,3,4,3,2,4,2]
Output: true
  • 1 <= nums.length <= 105
  • -109 <= nums[i] <= 109

My Solution

This solution is O(n)O(n) time and space complexity. I iterate through nums and store each unique value in a Map. If I encounter a value is already in the Map, I return true. If I add every item to the Map without encountering a duplicate, I return false.

const containsDuplicate = nums => {
	const map = new Map();
	for (let i = 0; i < nums.length; i++) {
		if (map.has(nums[i])) {
			return true;
		map.set(nums[i], i);
	return false;

Another solution would be to create a Set from nums and compare the sizes to each other. I initially chose the Map approach, as using a Set gaurantees traversing the entire nums array without the ability to return early. I’m not the only one who thought this, and found this useful comparison (with Objects instead of Maps) here.

const containsDuplicate = nums => {
	const set = new Set(nums);
	return set.size !== nums.length;