Leetcode - 217. Contains Duplicate

Easy

May 23, 2025

#arrays

#hashing

217. Contains Duplicate

Problem

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

Explanation:

The element 1 occurs at the indices 0 and 3.

Example 2:

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

Output: false

Explanation:

All elements are distinct.

Example 3:

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

Output: true

Constraints:

1 <= nums.length <= 105
-109 <= nums[i] <= 109

Approach

Simply use a hash map to keep track of which numbers we've already seen. If we've already seen a number, return true. Otherwise add to the hashmap and continue iterating. If we reach the end, there are no duplicates.

Complexity

Time: O(n)

We iterate through the array once

Space: O(n)

We create a hash map that can, at most, contain all the numbers

Solution

func containsDuplicate(nums []int) bool {
	seen := make(map[int]bool)
	for _, num := range nums {
		if _, exists := seen[num]; exists {
			return true
		}
		seen[num] = true
	}
	return false
}