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.

Example 1:

Input: [1,2,3,1]
Output: true

Example 2:

Input: [1,2,3,4]
Output: false

Example 3:

Input: [1,1,1,3,3,4,3,2,4,2]
Output: true

// Brute force
bool containsDuplicate(vector<int>& nums) { // time: O(n^2); space: O(1)
    int n = nums.size();
    for (int i = 0; i < n; ++i) {
        for (int j = i + 1; j < n; ++j) {
            if (nums[i] == nums[j]) return true;
        }
    }
    return false;
}

// Sort
bool containsDuplicate(vector<int>& nums) { // time: O(nlogn); space: O(1)
    sort(nums.begin(), nums.end());
    for (int i = 1; i < nums.size(); ++i) {
        if (nums[i] == nums[i - 1]) return true;
    }
    return false;
}

// Hash set
bool containsDuplicate(vector<int>& nums) { // time: O(n); space: O(n)
    unordered_set<int> st;
    for (int num : nums) {
        if (st.count(num)) return true;
        st.insert(num);
    }
    return false;
}