442. Find All Duplicates in an Array

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Problem

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.

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: []

Code

565. Array Nesting

class Solution {
    public List<Integer> findDuplicates(int[] nums) {     
        List<Integer> res = new ArrayList<>();
        
        for(int i = 0; i < nums.length; i++) {
            int index = Math.abs(nums[i]) - 1;
            if(nums[index] < 0) {
                res.add(index + 1);
            } else {
                nums[index] = -nums[index];
            }
        }
        
        return res;
    }
}