feat: add question 21

Author: yinxin630

21.合并两个有序链表.js

@@ -0,0 +1,68 @@
+/*
+ * @lc app=leetcode.cn id=21 lang=javascript
+ *
+ * [21] 合并两个有序链表
+ */
+
+// @lc code=start
+/**
+ * Definition for singly-linked list.
+ * function ListNode(val) {
+ *     this.val = val;
+ *     this.next = null;
+ * }
+ */
+/**
+ * @param {ListNode} l1
+ * @param {ListNode} l2
+ * @return {ListNode}
+ */
+var mergeTwoLists = function(l1, l2) {
+    let result = null;
+    let current = null;
+
+    while (l1 && l2) {
+        let next = l1.val <= l2.val ? l1 : l2;
+        if (!result) {
+            result = next;
+            current = next;
+        } else {
+            current.next = next;
+            current = next;
+        }
+
+        if (l1 === next) {
+            l1 = l1.next;
+        } else {
+            l2 = l2.next;
+        }
+    }
+
+    while (l1) {
+        if (!result) {
+            result = l1;
+            current = l1;
+        } else {
+            current.next = l1;
+            current = l1;
+        }
+
+        l1 = l1.next;
+    }
+
+    while (l2) {
+        if (!result) {
+            result = l2;
+            current = l2;
+        } else {
+            current.next = l2;
+            current = l2;
+        }
+
+        l2 = l2.next;
+    }
+
+    return result;
+};
+// @lc code=end
+