probelm: 0234 palindrom linked list

0234 Palindrome Linked List/0234 Palindrome Linked List.md

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+# 0234. Palindrome Linked List
+
+* Difficulty: easy
+* Link: https://leetcode.com/problems/palindrome-linked-list/
+* Topics: Linked-List
+
+# Clarification
+
+1. Check the inputs and outputs
+    - INPUT: Linked List
+    - OUTPUT: boolean
+2. Check the main goal
+
+# Naive Solution
+
+### Thought Process
+
+1. 把 linked list 轉成 array
+2. 再運用 index 進行比對
+- 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 isPalindrome(self, head: Optional[ListNode]) -> bool:
+            """
+            把 linked list 轉成 array
+            再運用 index 進行比對
+            """
+            array = []
+            current = head
+            while current != None:
+                array.append(current.val)
+                current = current.next
+
+            length = len(array)
+            for i in range(length // 2):
+                if array[i] != array[length - 1 - i]: return False
+
+            return True
+    ```
+
+
+### Complexity
+
+- Time complexity:$O(n)$
+
+    ![Untitled](./Untitled.png)
+
+- Space complexity:$O(n)$
+
+    ![Untitled](./Untitled%201.png)
+
+
+### Problems & Improvement
+
+- **Follow up:** Could you do it in `O(n)` time and `O(1)` space?
+
+# Improvement
+
+### Thought Process
+
+1. use slow, fast algorithm to find the middle node
+2. reverse the list from middle to end
+3. both go through from beginning and reverse list
+
+    if all node value is the same: true
+
+    else: false
+
+- 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 isPalindrome(self, head: Optional[ListNode]) -> bool:
+            """
+            1. use slow, fast algorithm to find the middle node
+            2. reverse the list from middle to end
+            3. both go through from beginning and reverse list
+                if all node value is the same: true
+                else: false
+            """
+
+            # find middle node
+            slow = head
+            fast = head
+            while fast and fast.next:
+                slow = slow.next
+                fast = fast.next.next
+
+            # reverse from middle to tail
+            prev = None
+            while slow:
+                tmp = slow.next
+                slow.next = prev
+                prev = slow
+                slow = tmp
+
+            # check whether palindrome
+            left = head
+            right = prev
+            while right:
+                if left.val != right.val:
+                    return False
+                left = left.next
+                right = right.next
+            return True
+    ```
+
+
+### Complexity
+
+- Time complexity:$O(n)$
+- Space complexity:$O(1)$
+
+# Check special cases, check error
+
+- 
+
+# Note
+
+- Reverse Linked List
+
+    ![Untitled](./Untitled%202.png)
+
+- [Palindrome Linked List - Leetcode 234 - Python](https://www.youtube.com/watch?v=yOzXms1J6Nk)