ContainsDuplicate.java

class Solution {
    public boolean containsDuplicate(int[] nums) {
      // Using set because it can retreive values in O(1) time. 
        Set<Integer> set = new HashSet<>();
        for(int i = 0; i< nums.length; i++) {
            if (set.contains(nums[i])) {
                return true; // found duplicate
            } else {
                set.add(nums[i]);
            }
        }
        return false;
    }
}