Reference: LeetCode
Difficulty: Easy

Problem

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: 

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

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

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

Analysis

Brute-Force

Skip.

Time: $O(N^2)$
Space: $O(1)$

Hash Set

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public boolean containsDuplicate(int[] nums) {
  Set<Integer> seen = new HashSet<>();
  for (int val : nums) {
    if (seen.contains(val)) return true;
    seen.add(val);
  }
  return false;
}

Time: $O(N)$
Space: $O(N)$

Sorting

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public boolean containsDuplicate(int[] nums) {
  if (nums.length == 0 || nums.length == 1) {
    return false;
  }
  int n = nums.length;
  Arrays.sort(nums);
  for (int i = 0; i < n - 1; ++i) {
    if (nums[i] == nums[i + 1]) return true;
  }
  return false;
}

Time: $O(N\log{N})$
Space: $O(1)$