217 Contains Duplicate

Given an array of integers, find if the array contains any duplicates. Your function should return true if any value appears at least twice in the array, and it should return false if every element is distinct.

The Idea: Use a hash set to confirm duplicates.

Complexity: O(n) time and space

bool containsDuplicate(vector<int>& nums) {
    if (nums.empty()) return false;

    unordered_set<int> unique;
    unique.reserve(nums.size());

    for (int i : nums) {
        if (unique.find(i) == unique.end()) 
            unique.insert(i);
        else return true;
    }

    return false;
}