Contains Duplicate

The problem

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

Comparing every pair is O(n²), and sorting first to spot neighbours is O(n log n). Both do more work than the question needs — all I really have to know is whether I've encountered a value before.

The approach

Keep a set of the values seen so far. For each number, if it's already in the set I've found a duplicate and can stop immediately.

Otherwise add it and continue. The early return means the best case is fast, and if the array is fully distinct I simply fall through to false after one pass.

The solution

function containsDuplicate(nums: number[]): boolean {
  const seen = new Set<number>();
  for (const n of nums) {
    if (seen.has(n)) return true;
    seen.add(n);
  }
  return false;
}

Time O(n)Space O(n)