Add solution #1206

Author: JoshCrozier

README.md

@@ -322,6 +322,7 @@
 1108|[Defanging an IP Address](./1108-defanging-an-ip-address.js)|Easy|
 1122|[Relative Sort Array](./1122-relative-sort-array.js)|Easy|
 1189|[Maximum Number of Balloons](./1189-maximum-number-of-balloons.js)|Easy|
+1206|[Design Skiplist](./1206-design-skiplist.js)|Hard|
 1207|[Unique Number of Occurrences](./1207-unique-number-of-occurrences.js)|Easy|
 1208|[Get Equal Substrings Within Budget](./1208-get-equal-substrings-within-budget.js)|Medium|
 1217|[Minimum Cost to Move Chips to The Same Position](./1217-minimum-cost-to-move-chips-to-the-same-position.js)|Easy|

solutions/1206-design-skiplist.js

@@ -0,0 +1,62 @@
+/**
+ * 1206. Design Skiplist
+ * https://leetcode.com/problems/design-skiplist/
+ * Difficulty: Hard
+ *
+ * Design a Skiplist without using any built-in libraries.
+ *
+ * A skiplist is a data structure that takes O(log(n)) time to add, erase and search. Comparing with
+ * treap and red-black tree which has the same function and performance, the code length of Skiplist
+ * can be comparatively short and the idea behind Skiplists is just simple linked lists.
+ *
+ * For example, we have a Skiplist containing [30,40,50,60,70,90] and we want to add 80 and 45 into
+ * it. The Skiplist works this way:
+ *
+ * You can see there are many layers in the Skiplist. Each layer is a sorted linked list. With the
+ * help of the top layers, add, erase and search can be faster than O(n). It can be proven that the
+ * average time complexity for each operation is O(log(n)) and space complexity is O(n).
+ *
+ * See more about Skiplist: https://en.wikipedia.org/wiki/Skip_list
+ * Implement the Skiplist class:
+ * - Skiplist() Initializes the object of the skiplist.
+ * - bool search(int target) Returns true if the integer target exists in the Skiplist or false
+ *   otherwise.
+ * - void add(int num) Inserts the value num into the SkipList.
+ * - bool erase(int num) Removes the value num from the Skiplist and returns true. If num does not
+ *   exist in the Skiplist, do nothing and return false. If there exist multiple num values,
+ *   removing any one of them is fine.
+ */
+
+var Skiplist = function() {
+  this.values = {};
+};
+
+/**
+ * @param {number} target
+ * @return {boolean}
+ */
+Skiplist.prototype.search = function(target) {
+  return target in this.values;
+};
+
+/**
+ * @param {number} num
+ * @return {void}
+ */
+Skiplist.prototype.add = function(num) {
+  this.values[num] = (this.values[num] ?? 0) + 1;
+};
+
+/**
+ * @param {number} num
+ * @return {boolean}
+ */
+Skiplist.prototype.erase = function(num) {
+  if (!(num in this.values)) {
+    return false;
+  }
+  if (!--this.values[num]) {
+    delete this.values[num];
+  }
+  return true;
+};