Given an integer array nums, return true if any value appears at least twice in the array, and return false if every element is distinct.

Example 1:

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

Example 2:

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

Example 3:

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

->如果有重複兩次，回傳true

<解題>

1. 使用迴圈及HashSet看有無重複，有的話就返回true，沒有的的話就加到HashSet中

class Solution {
    public boolean containsDuplicate(int[] nums) {
        HashSet<Integer> set = new HashSet<>();
        for (int i = 0; i < nums.length; i++) {
                if (set.contains(nums[i])) {
                    return true;
                }
                set.add(nums[i]);
        }
        return false;
    }
}

Time complexity: O(n) Space complexity: O(n)