Contains Duplicate

Approach

Use a hash set with a single pass:

1. Create an empty Set<number> named seen.
2. Iterate through each number in nums.
3. If the current number already exists in seen, return true immediately.
4. Otherwise, add the number to seen and continue.
5. If the loop finishes without finding a repeat, return false.

Why this works:

• seen stores every unique number encountered so far.
• The first time a value repeats, seen.has(value) detects it in constant average time.

Implementation

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

Complexity

• Time O(n): single pass over nums, with average O(1) set lookup/insert per element.
• Space O(n): in the worst case (all unique values), the set stores every number once.

Test Coverage

The test suite validates:

• LeetCode baseline examples
• Empty and single-element inputs
• Negative values and zeros
• Duplicates that appear far apart
• Unique unsorted arrays
• Large deterministic inputs (10k values)
• Input non-mutation behavior