Contains Duplicate

The question does not care where copies sit or how many times a duplicate repeats; it only asks if the count of unique values is strictly less than the total count. Tracking seen keys in an unordered hash set detects collisions the moment a repeated number is encountered, short-circuiting the remainder of the array.

Full solution

Hash set membership test: one pass, early-exit on the first repeat. The nested-loop and sort-based approaches both work but are strictly worse here — O(n) time and space beats them without the input-mutation tradeoff sorting requires.

def contains_duplicate(nums: list[int]) -> bool:
    seen: set[int] = set()
    for num in nums:
        if num in seen:
            return True
        seen.add(num)
    return False

function containsDuplicate(nums) {
  const seen = new Set();
  for (const num of nums) {
    if (seen.has(num)) return true;
    seen.add(num);
  }
  return false;
}

The theory behind it

Arrays — the ground this problem stands on. All Arrays problems

What Arrays is

An array is a row of fixed boxes laid side by side in computer memory, like numbered lockers in a hallway. Because each box occupies identical space and sits directly next to its neighbors, jumping to locker zero or locker ten thousand takes the exact same tiny fraction of time. Every box holds an item of the same type, addressed by an offset number called an index.

When to reach for it

Reach for an array when items arrive in a known sequence and need immediate retrieval by position number. Problems asking for running totals, prefix accumulations, cyclic rotations, or in-place rearrangements signal array mechanics. Whenever constraints require constant-time random lookups or contiguous cache scans across fixed collections, a flat sequence is the default container.

How the pattern works

Visualize a tape with zero-indexed slots stretching from start to end. Keep track of write and read cursors when modifying contents without allocating helper buffers. For running computations, maintain an invariant such as having processed all elements left of the current index while pending elements wait to the right. When modifying entries in place, consider scanning backwards from the end so unread data is not overwritten.