Add solution #21

Author: JoshCrozier

README.md

@@ -21,6 +21,7 @@
 13|[Roman to Integer](./0013-roman-to-integer.js)|Easy|
 14|[Longest Common Prefix](./0014-longest-common-prefix.js)|Easy|
 17|[Letter Combinations of a Phone Number](./0017-letter-combinations-of-a-phone-number.js)|Medium|
+21|[Merge Two Sorted Lists](./0021-merge-two-sorted-lists.js)|Easy|
 27|[Remove Element](./0027-remove-element.js)|Easy|
 31|[Next Permutation](./0031-next-permutation.js)|Medium|
 36|[Valid Sudoku](./0036-valid-sudoku.js)|Medium|

solutions/0021-merge-two-sorted-lists.js

@@ -0,0 +1,35 @@
+/**
+ * 21. Merge Two Sorted Lists
+ * https://leetcode.com/problems/merge-two-sorted-lists/
+ * Difficulty: Easy
+ *
+ * Merge two sorted linked lists and return it as a sorted list.
+ * The list should be made by splicing together the nodes of the
+ * first two lists.
+ */
+
+/**
+ * Definition for singly-linked list.
+ * function ListNode(val, next) {
+ *     this.val = (val===undefined ? 0 : val)
+ *     this.next = (next===undefined ? null : next)
+ * }
+ */
+/**
+ * @param {ListNode} l1
+ * @param {ListNode} l2
+ * @return {ListNode}
+ */
+var mergeTwoLists = function(l1, l2) {
+  if (!l1 || !l2) {
+    return l1 || l2;
+  }
+
+  if (l1.val > l2.val) {
+    [l2, l1] = [l1, l2];
+  }
+
+  l1.next = mergeTwoLists(l1.next, l2);
+
+  return l1;
+};