442. Find All Duplicates in an Array(Solution || Leetcode medium || Java)

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) {
        //As the range is 1 to n, hence, we will use cycle sort to
        // solve this question
        List<Integer> ans = new ArrayList<>();
      
        int i = 0;
        
        while(i < nums.length){//Applying cycle sort
            int correct = nums[i] - 1;
            
            if(nums[i] != nums[correct]){
                int temp = nums[i];
                nums[i] = nums[correct];
                nums[correct] = temp;
            }
            else{
                i++;
            }
        }
        
        for(int j = 0; j < nums.length; j++){//Adding duplicates
            if(nums[j] != j+1){
                ans.add(nums[j]);
            }
        }
        
        return ans;
    }
}

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