217. Contains Duplicate

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

Constraints:
1 <= nums.length <= 105
-109 <= nums[i] <= 109

Intuition

First thoughts

In order to solve this question in a brute-force manner, we’ll have to:

• loop through all the elements and for each element
  • we need to check against all the remaining elements if that element is present or not.

This approach has:

• run time complexity of O(n**2) (as we have nested loops)
• space complextity of O(1) (since we don’t need to allocate any extra storage)

Further thoughts

Can we do better than O(n**2)?

• Well, if we use hash set to store all the elements of the given array, then we should be able to check the existence of any element in constant time.
• Thus, we can just loop through all the elements and then for each element we can check in constant time whether the element exists or not.

Final approach

• Initialize an empty hashset by name lookup
• Loop through all the elements of given array
  • if that element is not present in lookup, then let’s add it and move ahead
  • otherwise, (i.e. element already exists, duplicated spotted!)
    • return True
• return False, as we exhausted the given array and didn’t find any duplicate.
• Time complexity: O(n) (since we only process elements in a single loop)
• Space complexity: O(n) (since we create a new hashset to store all the elements of given array)

Python code

class Solution:
    def containsDuplicate(self, nums: List[int]) -> bool:
        lookup = set()
        for x in nums:
            if x in lookup:
                return True
            lookup.add(x)
        return False