Add solution #2007

Author: JoshCrozier

README.md

@@ -1,4 +1,4 @@
-# 1,683 LeetCode solutions in JavaScript
+# 1,684 LeetCode solutions in JavaScript
 
 [https://leetcodejavascript.com](https://leetcodejavascript.com)
 
@@ -1539,6 +1539,7 @@
 2002|[Maximum Product of the Length of Two Palindromic Subsequences](./solutions/2002-maximum-product-of-the-length-of-two-palindromic-subsequences.js)|Medium|
 2003|[Smallest Missing Genetic Value in Each Subtree](./solutions/2003-smallest-missing-genetic-value-in-each-subtree.js)|Hard|
 2006|[Count Number of Pairs With Absolute Difference K](./solutions/2006-count-number-of-pairs-with-absolute-difference-k.js)|Easy|
+2007|[Find Original Array From Doubled Array](./solutions/2007-find-original-array-from-doubled-array.js)|Medium|
 2011|[Final Value of Variable After Performing Operations](./solutions/2011-final-value-of-variable-after-performing-operations.js)|Easy|
 2016|[Maximum Difference Between Increasing Elements](./solutions/2016-maximum-difference-between-increasing-elements.js)|Easy|
 2017|[Grid Game](./solutions/2017-grid-game.js)|Medium|

solutions/2007-find-original-array-from-doubled-array.js

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+/**
+ * 2007. Find Original Array From Doubled Array
+ * https://leetcode.com/problems/find-original-array-from-doubled-array/
+ * Difficulty: Medium
+ *
+ * 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.
+ */
+
+/**
+ * @param {number[]} changed
+ * @return {number[]}
+ */
+var findOriginalArray = function(changed) {
+  if (changed.length % 2 !== 0) return [];
+
+  const frequency = new Map();
+  const result = [];
+
+  changed.sort((a, b) => a - b);
+
+  for (const num of changed) {
+    frequency.set(num, (frequency.get(num) || 0) + 1);
+  }
+
+  for (const num of changed) {
+    if (frequency.get(num) === 0) continue;
+
+    frequency.set(num, frequency.get(num) - 1);
+
+    const doubled = num * 2;
+    if (!frequency.has(doubled) || frequency.get(doubled) === 0) return [];
+
+    frequency.set(doubled, frequency.get(doubled) - 1);
+    result.push(num);
+  }
+
+  return result;
+};