Contains Duplicate

Problem

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

Constraints

• 1 ≤ nums.length ≤ 10⁵
• −10⁹ ≤ nums[i] ≤ 10⁹

Approach — Hashing & Counting

This is a Hashing & Counting problem. The idea: store what you've seen in a hash map for O(1) lookups, trading a little space to avoid a nested scan. Work through the reference code below line by line, then re-derive it yourself in the editor — that's how the pattern sticks.

Complexity: O(n) time, O(n) space.

Solution code

Python

class Solution:
    def containsDuplicate(self, nums):
        return len(set(nums)) != len(nums)

Java

class Solution {
    public boolean containsDuplicate(int[] nums) {
        Set<Integer> seen = new HashSet<>();
        for (int num : nums) {
            if (!seen.add(num)) {
                return true;
            }
        }
        return false;
    }
}

Practice it

Reading a solution isn't the same as being able to write it under pressure. Open this problem in the in-browser editor, hide the solution, and solve it from scratch — your code runs against real test cases instantly.