Merge Two Sorted Lists

Author: w2xi

README.md

@@ -7,4 +7,5 @@
 |7|[整数反转](https://leetcode-cn.com/problems/reverse-integer/)|[JavaScript](./algorithms/reverse-integer.js)|Medium|
 |9|[回文数](https://leetcode-cn.com/problems/palindrome-number/)|[JavaScript](./algorithms/palindrome-number.js)|Easy|
 |14|[最长公共前缀](https://leetcode-cn.com/problems/longest-common-prefix/)|[JavaScript](./algorithms/longest-common-prefix.js)|Easy|
+|21|[合并两个有序链表](https://leetcode-cn.com/problems/merge-two-sorted-lists/)|[JavaScript](./algorithms/merge-two-sorted-lists.js)|Easy|
 |189|[轮转数组](https://leetcode-cn.com/problems/rotate-array/)|[JavaScript](./algorithms/rotate-array.js)|Medium|

algorithms/merge-two-sorted-lists.js

@@ -0,0 +1,53 @@
+/**
+ * Definition for singly-linked list.
+ * function ListNode(val, next) {
+ *     this.val = (val===undefined ? 0 : val)
+ *     this.next = (next===undefined ? null : next)
+ * }
+ */
+/**
+ * @param {ListNode} list1
+ * @param {ListNode} list2
+ * @return {ListNode}
+ */
+var mergeTwoLists = function (list1, list2) {
+  let dummyHead = new ListNode(0);
+  let current = dummyHead;
+
+  while (list1 && list2) {
+    if (list1.val < list2.val) {
+      current.next = list1;
+      current = current.next;
+      list1 = list1.next;
+    } else {
+      current.next = list2;
+      current = current.next;
+      list2 = list2.next;
+    }
+  }
+
+  if (!list1) {
+    current.next = list2;
+  }
+
+  if (!list2) {
+    current.next = list1;
+  }
+
+  return dummyHead.next;
+};
+
+
+// recursion version
+var mergeTwoLists = function (list1, list2) {
+  if (!list1) return list2;
+  if (!list2) return list1;
+
+  if (list1.val < list2.val) {
+    list1.next = mergeTwoLists(list1.next, list2);
+    return list1;
+  } else {
+    list2.next = mergeTwoLists(list1, list2.next);
+    return list2;
+  }
+}