// Run-time complexity O(nlogn)
    // Memory complexity O(1)
    bool containsDuplicate(vector<int>& nums) {

        if (nums.empty())
            return false;

        sort(nums.begin(), nums.end());
        for (int i = 0; i < nums.size() - 1; ++i) {
            if (nums[i] == nums[i + 1]) {
                return true;
            }
        }
        return false;
    }

    // Run-time complexity O(n)
    // Memory complexity O(n)
    bool containsDuplicate(vector<int>& nums) {
        unordered_set<int> hashset;
        for (int num : nums) {
            if (hashset.count(num))
                return true;
            hashset.insert(num);
        }
        return false;
    }