add 21 merge linked list

Author: teckoo

_posts/coding/leetcode/2021-01-01-21-merge-two-sorted-lists.md

@@ -0,0 +1,75 @@
+---
+layout: leetcode
+title: "21. Merge Two Sorted Lists"
+categories: [leetcode]
+---
+
+[Leetcode Link](https://leetcode.com/problems/merge-two-sorted-lists/)
+
+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.
+
+ 
+```
+Example 1:
+
+
+Input: l1 = [1,2,4], l2 = [1,3,4]
+Output: [1,1,2,3,4,4]
+
+Example 2:
+
+Input: l1 = [], l2 = []
+Output: []
+
+Example 3:
+
+Input: l1 = [], l2 = [0]
+Output: [0]
+ 
+
+Constraints:
+
+The number of nodes in both lists is in the range [0, 50].
+-100 <= Node.val <= 100
+Both l1 and l2 are sorted in non-decreasing order.
+```
+
+# Solution
+
+## Iteration
+
+T: O(m + n), S:O(1)
+
+```python
+def mergeTwoLists(self, l1, l2):
+  prehead = ListNode(-1)
+
+  curr = prehead
+  while l1 and l2:
+    if l1.val < l2.val:
+      curr.next = l1
+      l1 = l1.next
+    else:
+      curr.next = l2
+      l2 = l2.next
+    curr = curr.next
+  curr.next = l1 or l2
+
+  return prehead.next
+```
+
+## Recursion
+
+T: O(m + n), S:O(m + n)
+```python
+def mergeTwoList(self, l1, l2):
+  if not l1: return l2
+  if not l2: return l1
+
+  if l1.val < l2.val:
+    l1.next = self.mergeTwoList(l1.next, l2)
+    return l1
+  else:
+    l2.next = self.mergeTwoList(l1, l2.next)
+    return l2
+```

media/js/tag-search-data.js

@@ -71,7 +71,15 @@ var problems = {
         notes: "hashmap, T: O(N^3), S: O(N)",
         templates: [],
     },
-    "22": {
+    "21": {
+        name: "21. Merge Two Sorted Lists",
+        url: "/leetcode/21-merge-two-sorted-lists",
+        tags: ["sort", "linked_list"],
+        keywords: ["merge_sort", "lv_1"],
+        templates: ["merge_sort"],
+        notes: "Use dummy head",
+    },
+     "22": {
         name: "22. Generate Parentheses",
         url: "/leetcode/22-generate-parentheses",
         tags: ["backtrack", "array"],