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442. Find All Duplicates in an Array

Given an integer array nums of length n where all the integers of nums are in the range [1, n] and each integer appears once or twice, return an array of all the integers that appears twice**.

You must write an algorithm that runs in O(n)time and uses only constant extra space.

Example 1:

Input: nums = [4,3,2,7,8,2,3,1]
Output: [2,3]

Example 2:

Input: nums = [1,1,2]
Output: [1]

Example 3:

Input: nums = [1]
Output: []

Constraints:

  • n == nums.length
  • 1 <= n <= 105
  • 1 <= nums[i] <= n
  • Each element in nums appears once or twice.

Solution:

class Solution {
    public List<Integer> findDuplicates(int[] nums) {
        Set<Integer> set = new HashSet<Integer>();
        List<Integer> result = new ArrayList<Integer>();
        for (int i = 0; i < nums.length; i++){
            if (set.contains(nums[i])){
                result.add(nums[i]);
            }
            set.add(nums[i]);
        }
        return result;

    }
}

// TC: O(n)
// SC: O(n)