Create 0234-palindrome-linked-list.md

Added solution to Leetcode 234 problem

Author: PradnyaGaitonde

dsa-solutions/lc-solutions/0200-0299/0234-palindrome-linked-list.md

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+---
+id: palindrome-linked-list
+title: Palindrome Linked List(LeetCode)
+sidebar_label: 0234-Palindrome Linked List
+tags:
+  - Linked List
+  - Two Pointer
+  - Stack
+  - Recursion
+description: Given the head of a singly linked list, return true if it is a palindrome or false otherwise.
+---
+
+## Problem Statement
+
+Given the `head` of a singly linked list, return `true` if it is a palindrome or `false` otherwise.
+
+### Examples
+
+**Example 1:**
+
+![image](https://github.com/PradnyaGaitonde/codeharborhub.github.io/assets/116059908/3912ff1d-6425-4b30-aa4c-94955c9ee9bd)
+
+```plaintext
+Input: head = [1,2,2,1]
+Output: true
+```
+
+**Example 2:**
+
+![image](https://github.com/PradnyaGaitonde/codeharborhub.github.io/assets/116059908/429d0cde-46de-4c2d-af7a-dd69bda5c426)
+
+```plaintext
+Input: head = [1,2]
+Output: false
+```
+
+### Constraints
+
+- The number of nodes in the list is in the range `[1, 105]`.
+- `0 <= Node.val <= 9`
+
+## Solution
+
+### Approach
+
+To check if a linked list is a palindrome using O(1) extra space, we can reverse the second half of the linked list and then compare it to the first half. The key steps involve:
+
+1. Finding the middle of the linked list.
+   
+   ![image](https://github.com/PradnyaGaitonde/codeharborhub.github.io/assets/116059908/5bfdbfd5-0f6f-4040-a946-9820621e2910)
+
+2. Reversing the second half of the list.
+
+   ![image](https://github.com/PradnyaGaitonde/codeharborhub.github.io/assets/116059908/5a2e1fdb-d8ca-4842-8638-23a1f0d3e2d5)
+   
+3. Comparing the two halves for equality.
+
+   ![image](https://github.com/PradnyaGaitonde/codeharborhub.github.io/assets/116059908/ce99042e-62e4-4bf5-92ec-2fb01db9fc2d)
+
+(Note: This process works regardless of whether the length of the linked list is odd or even, as the comparison will stop when slow reaches the "dead-end" node.)
+
+  ![image](https://github.com/PradnyaGaitonde/codeharborhub.github.io/assets/116059908/7f423edf-044a-40f4-bccf-d762d65a66a9)
+
+#### Algorithm
+
+1. Find the middle of the linked list:
+* Use two pointers, `slow` and `fast`. Move `slow` by one step and `fast` by two steps in each iteration. When `fast` reaches the end, slow will be at the middle.
+2. Reverse the second half:
+* Starting from the middle node, reverse the direction of the `next` pointers for each node until the end of the list.
+3. Compare the first and second halves:
+* Initialize two pointers, one at the head and the other at the start of the reversed second half.
+* Compare the values of the nodes pointed to by these pointers. If any values differ, the list is not a palindrome. If all values match, the list is a palindrome.
+
+#### Implementation
+
+Javascript Code:
+
+```Javascript
+var isPalindrome = function(head) {
+    let slow = head, fast = head, prev = null, temp;
+    while (fast && fast.next) {
+        slow = slow.next;
+        fast = fast.next.next;
+    }
+    prev = slow;
+    slow = slow.next;
+    prev.next = null;
+    while (slow) {
+        temp = slow.next;
+        slow.next = prev;
+        prev = slow;
+        slow = temp;
+    }
+    fast = head;
+    slow = prev;
+    while (slow) {
+        if (fast.val !== slow.val) return false;
+        fast = fast.next;
+        slow = slow.next;
+    }
+    return true;
+};
+```
+
+Python Code:
+
+```Python
+class Solution:
+    def isPalindrome(self, head: ListNode) -> bool:
+        slow, fast, prev = head, head, None
+        while fast and fast.next:
+            slow, fast = slow.next, fast.next.next
+        prev, slow, prev.next = slow, slow.next, None
+        while slow:
+            slow.next, prev, slow = prev, slow, slow.next
+        fast, slow = head, prev
+        while slow:
+            if fast.val != slow.val:
+                return False
+            fast, slow = fast.next, slow.next
+        return True
+```
+
+Java Code:
+
+```Java
+class Solution {
+    public boolean isPalindrome(ListNode head) {
+        ListNode slow = head, fast = head, prev = null, temp;
+        while (fast != null && fast.next != null) {
+            slow = slow.next;
+            fast = fast.next.next;
+        }
+        prev = slow;
+        slow = slow.next;
+        prev.next = null;
+        while (slow != null) {
+            temp = slow.next;
+            slow.next = prev;
+            prev = slow;
+            slow = temp;
+        }
+        fast = head;
+        slow = prev;
+        while (slow != null) {
+            if (fast.val != slow.val) return false;
+            fast = fast.next;
+            slow = slow.next;
+        }
+        return true;
+    }
+}
+```
+
+C++ Code:
+
+```C++
+class Solution {
+public:
+    bool isPalindrome(ListNode* head) {
+        ListNode *slow = head, *fast = head, *prev = nullptr, *temp;
+        while (fast && fast->next) {
+            slow = slow->next;
+            fast = fast->next->next;
+        }
+        prev = slow;
+        slow = slow->next;
+        prev->next = NULL;
+        while (slow) {
+            temp = slow->next;
+            slow->next = prev;
+            prev = slow;
+            slow = temp;
+        }
+        fast = head;
+        slow = prev;
+        while (slow) {
+            if (fast->val != slow->val) return false;
+            fast = fast->next;
+            slow = slow->next;
+        }
+        return true;
+    }
+};
+```
+
+### Complexity Analysis
+
+- **Time complexity**: O(N)
+- **Space complexity**: O(1)
+
+### Conclusion
+
+By using two pointers to find the middle, reversing the second half of the linked list, and then comparing the two halves, we can determine if a linked list is a palindrome with a time complexity of O(N) and a space complexity of O(1). This approach ensures we do not use extra space beyond the input list itself, meeting the problem's constraints.