Free practice lab

Code Comparison

Compare two implementations of the same task and justify which is better under the stated constraints. Free — no account needed.

beginnerjavascript ~5 minFree

Detect duplicates: nested loops vs a Set

Two correct ways to check an array for duplicate integers, with very different costs at scale.

Write containsDuplicate(nums) that returns true if any integer appears more than once in nums, otherwise false.

Constraints

nums contains between 0 and 100,000 integers.
Values range from -1,000,000,000 to 1,000,000,000.
The function runs in a request handler, so response time matters.
Memory is not tightly limited: a few megabytes of extra memory is acceptable.

Code A · javascript

1function containsDuplicate(nums) {
2  for (let i = 0; i < nums.length; i++) {
3    for (let j = i + 1; j < nums.length; j++) {
4      if (nums[i] === nums[j]) return true;
5    }
6  }
7  return false;
8}

Code B · javascript

1function containsDuplicate(nums) {
2  const seen = new Set();
3  for (const n of nums) {
4    if (seen.has(n)) return true;
5    seen.add(n);
6  }
7  return false;
8}

Representative test cases
Input	Expected	Note
[1, 2, 3, 1]	true	Duplicate at the ends
[1, 2, 3, 4]	false
[]	false	Empty input
[7]	false	Single element
[-1, 0, -1]	true	Negative values