Return true if any value appears at least twice in nums, and false if every element is distinct.
Remember every value seen. Seeing one again means a duplicate.
1function containsDuplicate(nums: number[]): boolean {2const seen = new Set<number>();3for (const x of nums) {4if (seen.has(x)) return true;5seen.add(x);6}7return false;8}
Start with an empty set.
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