problemhardalgorithmsleetcode-2122leetcode 2122leetcode2122

Recover the Original Array

HardUpdated: Aug 2, 2025
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LeetCode

Problem

Alice had a 0-indexed array arr consisting of n positive integers. She chose an arbitrary positive integer k and created two new 0-indexed integer arrays lower and higher in the following manner:

  1. lower[i] = arr[i] - k, for every index i where 0 <= i < n
  2. higher[i] = arr[i] + k, for every index i where 0 <= i < n

Unfortunately, Alice lost all three arrays. However, she remembers the integers that were present in the arrays lower and higher, but not the array each integer belonged to. Help Alice and recover the original array.

Given an array nums consisting of 2n integers, where exactly n of the integers were present in lower and the remaining in higher, return the original array arr. In case the answer is not unique, return any valid array.

Note: The test cases are generated such that there exists at least one valid array arr.

Examples

Example 1:

Input: nums = [2,10,6,4,8,12]
Output: [3,7,11]
Explanation:
If arr = [3,7,11] and k = 1, we get lower = [2,6,10] and higher = [4,8,12].
Combining lower and higher gives us [2,6,10,4,8,12], which is a permutation of nums.
Another valid possibility is that arr = [5,7,9] and k = 3. In that case, lower = [2,4,6] and higher = [8,10,12].

Example 2:

Input: nums = [1,1,3,3]
Output: [2,2]
Explanation:
If arr = [2,2] and k = 1, we get lower = [1,1] and higher = [3,3].
Combining lower and higher gives us [1,1,3,3], which is equal to nums.
Note that arr cannot be [1,3] because in that case, the only possible way to obtain [1,1,3,3] is with k = 0.
This is invalid since k must be positive.

Example 3:

Input: nums = [5,435]
Output: [220]
Explanation:
The only possible combination is arr = [220] and k = 215. Using them, we get lower = [5] and higher = [435].

Solution

Method 1 - Using 2 sets

Here is the approach:

  1. Create Sets:
    • Separate the nums array into two sets for lower and higher values.
  2. Identify Elements:
    • For each x in nums, test if (x + k) or (x - k) is present in the corresponding set.
  3. Construct the Original Array:
    • If (x + k) is present in the set, then the original element was x - k.
    • If (x - k) is present in the set, then the original element was x + k.

Code

Java
public class Solution {
    public int[] recoverArray(int[] nums, int k) {
        Set<Integer> lowerSet = new HashSet<>();
        Set<Integer> higherSet = new HashSet<>();
        
        for (int num : nums) {
            // Assume lower contains all n values
            lowerSet.add(num);
        }
        
        int[] result = new int[nums.length / 2];
        int idx = 0;
        
        for (int num : nums) {
            if (lowerSet.contains(num + k)) {
                result[idx++] = num + k;
                lowerSet.remove(num);
                lowerSet.remove(num + k);
            }
            if (lowerSet.contains(num - k)) {
                result[idx++] = num - k;
                lowerSet.remove(num);
                lowerSet.remove(num - k);
            }
        }
        
        return result;
    }

}
Python
class Solution: 
    def recover_array(nums, k):
    lower_set = set(nums)  # Assume lower contains all n values

    result = []

    for num in nums:
        if (num + k) in lower_set:
            result.append(num + k)
            lower_set.remove(num)
            lower_set.remove(num + k)
        if (num - k) in lower_set:
            result.append(num - k)
            lower_set.remove(num)
            lower_set.remove(num - k)

    return result

Complexity

  • Time: O(n), where n is the number of elements in arr. Each element is processed a constant number of times.
  • Space: O(n), where n is the number of elements in arr. This is used for storing the sets and the output array.

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