Easyhashing

Contains Duplicate

Problem (restated)

Return true if any value appears at least twice in the array.

Intuition

A set remembers seen values; a second sighting is a duplicate.

Approaches

Hash set

Verified

Time O(n)Space O(n)

Idea. Insert each number into a set; if already present, return true.

Walkthrough. [1,2,3,1] → see 1 again → true.

Trade-offs. Sorting is O(n log n) with O(1) extra space if allowed to mutate.

Solution

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

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

Reflection

Which cue made you pick this pattern in under 90 seconds?
What input would break a wrong invariant?