problem: 0021 merge two sorted lists

Author: cychiu8

0021 Merge Two Sorted Lists/0021 Merge Two Sorted Lists.md

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+# 0021. Merge Two Sorted Lists
+
+* Difficulty: easy
+* Link: https://leetcode.com/problems/merge-two-sorted-lists/
+* Topics: Linked-List
+
+# Clarification
+
+1. Check the inputs and outputs
+    - INPUT: two Linked List
+    - OUTPUT: one Linked List
+
+# Naive Solution
+
+### Thought Process
+
+- 使用 dummy node
+1. 兩個 pointer 分別指向兩個 linked list
+2. 將 next 指向 val 較小的那個 Node
+- Implement
+    
+    ```python
+    # Definition for singly-linked list.
+    # class ListNode:
+    #     def __init__(self, val=0, next=None):
+    #         self.val = val
+    #         self.next = next
+    class Solution:
+        def mergeTwoLists(self, list1: Optional[ListNode], list2: Optional[ListNode]) -> Optional[ListNode]:
+            dummy = ListNode(None)
+            prev = dummy
+            while list1 and list2:
+                if list1.val < list2.val:
+                    prev.next = list1
+                    list1 = list1.next
+                else:
+                    prev.next = list2
+                    list2 = list2.next
+                prev = prev.next
+            
+            if list1:
+                prev.next = list1
+            if list2:
+                prev.next = list2
+            return dummy.next
+    ```
+    
+
+### Complexity
+
+- Time complexity:$O(n)$
+    
+    ![Untitled](./Untitled.png)
+    
+- Space complexity:$O(1)$