Contains Duplicate

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Problem

Given an integer array, determine whether any value appears at least twice in the array. Return true if a duplicate exists, and false if every element is distinct.

Examples

Example 1

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

Output:true

The value 1 appears twice.

Example 2

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

Output:false

All elements are distinct.

Constraints

1 <= nums.length <= 10^5
-10^9 <= nums[i] <= 10^9

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How to approach it: the Arrays & Hashing pattern

Arrays give O(1) index access; hash maps and sets give O(1) average lookup, insert, and delete by trading space for speed. Together they're the toolkit for turning brute-force O(n²) comparisons into a single O(n) pass.

Look for this pattern when

You need to check "have I seen this value before?" quickly.
You're counting occurrences or grouping items by a derived key (e.g. sorted letters for anagrams).
A nested loop is comparing every pair — a hash map can often replace the inner loop.

Read the full Arrays & Hashing guide →

Video walkthroughs

Original problem on LeetCode ↗