Problem Description

Given an array nums of n integers where nums[i] is in the range [1, n], return an array of all the integers in the range [1, n] that do not appear in nums.

Approach

HashSet Approach (or the Time-Space Trade-off approach).

Solution

class Solution {
    public List<Integer> findDisappearedNumbers(int[] nums) {
        List<Integer> al = new ArrayList<>();
        HashSet<Integer> set = new HashSet<>();
        
        for (int num : nums) {
            set.add(num);
        }
        
        for (int i = 1; i <= nums.length; i++) {
            if (!set.contains(i)) {
                al.add(i);
            }
        }
        
        return al;
    }
}