LeetCode 442. Find All Duplicates in an Array

Description

https://leetcode.com/problems/find-all-duplicates-in-an-array/

Given an array of integers, 1 ≤ a[i] ≤ n (n = size of array), some elements appear twice and others appear once.

Find all the elements that appear twice in this array.

Could you do it without extra space and in O(n) runtime?

Example:

Input:
[4,3,2,7,8,2,3,1]

Output:
[2,3]

Explanation

Counter the numbers where occurs more than once.

Python Solution

class Solution:
    def findDuplicates(self, nums: List[int]) -> List[int]:
        counter = {}
        
        for num in nums:
            counter[num] = counter.get(num, 0) + 1
        
        results = []
        for key, value in counter.items():
            if value > 1:
                results.append(key)
                
                
        return results
            
  • Time Complexity: O(N).
  • Space Complexity: O(N).

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