Add solution #138

Author: JoshCrozier

README.md

@@ -85,6 +85,7 @@
 135|[Candy](./0135-candy.js)|Hard|
 136|[Single Number](./0136-single-number.js)|Easy|
 137|[Single Number II](./0137-single-number-ii.js)|Medium|
+138|[Copy List with Random Pointer](./0138-copy-list-with-random-pointer.js)|Medium|
 141|[Linked List Cycle](./0141-linked-list-cycle.js)|Easy|
 142|[Linked List Cycle II](./0142-linked-list-cycle-ii.js)|Medium|
 143|[Reorder List](./0143-reorder-list.js)|Medium|

solutions/0138-copy-list-with-random-pointer.js

@@ -0,0 +1,61 @@
+/**
+ * 138. Copy List with Random Pointer
+ * https://leetcode.com/problems/copy-list-with-random-pointer/
+ * Difficulty: Medium
+ *
+ * A linked list of length n is given such that each node contains an additional random pointer,
+ * which could point to any node in the list, or null.
+ *
+ * Construct a deep copy of the list. The deep copy should consist of exactly n brand new nodes,
+ * where each new node has its value set to the value of its corresponding original node. Both
+ * the next and random pointer of the new nodes should point to new nodes in the copied list such
+ * that the pointers in the original list and copied list represent the same list state. None of
+ * the pointers in the new list should point to nodes in the original list.
+ *
+ * For example, if there are two nodes X and Y in the original list, where X.random --> Y, then
+ * for the corresponding two nodes x and y in the copied list, x.random --> y.
+ *
+ * Return the head of the copied linked list.
+ *
+ * The linked list is represented in the input/output as a list of n nodes. Each node is
+ * represented as a pair of [val, random_index] where:
+ * - val: an integer representing Node.val
+ * - random_index: the index of the node (range from 0 to n-1) that the random pointer points
+ *   to, or null if it does not point to any node.
+ *
+ * Your code will only be given the head of the original linked list.
+ */
+
+/**
+ * // Definition for a Node.
+ * function Node(val, next, random) {
+ *    this.val = val;
+ *    this.next = next;
+ *    this.random = random;
+ * };
+ */
+
+/**
+ * @param {Node} head
+ * @return {Node}
+ */
+var copyRandomList = function(head) {
+  const map = new Map();
+
+  function copy(reference) {
+    if (reference === null) return null;
+    const instance = map.get(reference);
+    if (instance) {
+      return instance;
+    }
+
+    const node = new Node(reference.val);
+    map.set(reference, node);
+    node.next = copy(reference.next);
+    node.random = copy(reference.random);
+
+    return node;
+  }
+
+  return copy(head);
+};