2007. Find Original Array From Doubled Array

Description

An integer array original is transformed into a doubled array changed by appending twice the value of every element in original, and then randomly shuffling the resulting array.

Given an array changed, return original if changed is a doubled array. If changed is not a doubled array, return an empty array. The elements in original may be returned in any order.

 

Example 1:

Input: changed = [1,3,4,2,6,8]
Output: [1,3,4]
Explanation: One possible original array could be [1,3,4]:
- Twice the value of 1 is 1 * 2 = 2.
- Twice the value of 3 is 3 * 2 = 6.
- Twice the value of 4 is 4 * 2 = 8.
Other original arrays could be [4,3,1] or [3,1,4].

Example 2:

Input: changed = [6,3,0,1]
Output: []
Explanation: changed is not a doubled array.

Example 3:

Input: changed = [1]
Output: []
Explanation: changed is not a doubled array.

 

Constraints:

• 1 <= changed.length <= 105
• 0 <= changed[i] <= 105

 

Solutions

Solution: Greedy

• Time complexity: O(nlogn)
• Space complexity: O(n)

 

JavaScript

/**
 * @param {number[]} changed
 * @return {number[]}
 */
const findOriginalArray = function (changed) {
  const changedMap = changed.reduce((map, value) => {
    const count = map.get(value) ?? 0;

    return map.set(value, count + 1);
  }, new Map());
  const result = [];
  const values = [...changedMap.keys()].sort((a, b) => a - b);

  for (const value of values) {
    const count = changedMap.get(value);

    if (!count) continue;
    if (value === 0) {
      if (count % 2) return [];
      result.push(...Array.from({length: count / 2}).fill(0));
      continue;
    }
    if (changedMap.has(value * 2)) {
      const doubledCount = changedMap.get(value * 2);

      if (count > doubledCount) return [];
      result.push(...new Array(count).fill(value));
      changedMap.set(value * 2, doubledCount - count);
      continue;
    }
    return [];
  }
  return result;
};