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.
方法1： Sort
Time complexity: O(nlogn)
Space complexity: O(1)
方法2：Set
Time complexity: O(n)
Space complexity: O(n)
方法3：Brute Force
Time complexity: O(n^2)
Space complexity: O(1)
