Add Linked List Palindrome check

Python/LinkedListPalindromeCheck.py

@@ -0,0 +1,134 @@
+"""
+Problem - Check if a linked list is a palidrome or not
+
+Algorithm Explaination:
+1. Get the middle of the linked list.
+2. Reverse the second half of the linked list.
+3. Check if the first half and second half are identical.
+4. Construct the original linked list by reversing the second half again and attaching it back to the first half
+
+Time Complexity - O(n)
+Auxiliary Space Complexity - O(1)
+
+
+Example Test Cases - 
+1. I/P - ['a', 'b', 'a', 'c', 'a', 'b', 'a']
+    O/P - "Is Palindrome"
+2. I/P - ['a']
+    O/P - "Is Palindrome"
+3. I/P - ['a', 'b', 'c']
+    O/P - "Not Palindrome"
+"""
+
+class Node:
+    def __init__(self, data):
+
+        self.data = data
+        self.next = None
+
+
+class LinkedList:
+    def __init__(self):
+        self.head = None
+
+    def isPalindrome(self, head):
+        slow_ptr = head
+        fast_ptr = head
+        prev_of_slow_ptr = head
+
+        midnode = None
+
+        res = True
+
+        if (head != None and head.next != None):
+            while (fast_ptr != None and
+                   fast_ptr.next != None):
+
+                fast_ptr = fast_ptr.next.next
+                prev_of_slow_ptr = slow_ptr
+                slow_ptr = slow_ptr.next
+
+            if (fast_ptr != None):
+                midnode = slow_ptr
+                slow_ptr = slow_ptr.next
+
+            second_half = slow_ptr
+
+            prev_of_slow_ptr.next = None
+
+            second_half = self.reverse(second_half)
+
+            res = self.compareLists(head, second_half)
+
+            second_half = self.reverse(second_half)
+
+            if (midnode != None):
+                prev_of_slow_ptr.next = midnode
+                midnode.next = second_half
+            else:
+                prev_of_slow_ptr.next = second_half
+        return res
+
+    def reverse(self, second_half):
+
+        prev = None
+        current = second_half
+        next = None
+
+        while current != None:
+            next = current.next
+            current.next = prev
+            prev = current
+            current = next
+
+        second_half = prev
+        return second_half
+
+    def compareLists(self, head1, head2):
+
+        temp1 = head1
+        temp2 = head2
+
+        while (temp1 and temp2):
+            if (temp1.data == temp2.data):
+                temp1 = temp1.next
+                temp2 = temp2.next
+            else:
+                return 0
+
+        if (temp1 == None and temp2 == None):
+            return 1
+
+        return 0
+
+    def push(self, new_data):
+        new_node = Node(new_data)
+        new_node.next = self.head
+        self.head = new_node
+
+    def printList(self):
+
+        temp = self.head
+
+        while(temp):
+            print(temp.data, end="->")
+            temp = temp.next
+
+        print("NULL")
+
+
+# Driver code
+if __name__ == '__main__':
+
+    ll = LinkedList()
+    s = ['a','b','c']
+
+    for i in range(len(s)):
+        ll.push(s[i])
+    ll.printList()
+
+    if (ll.isPalindrome(ll.head) != False):
+        print("Is Palindrome\n")
+    else:
+        print("Not Palindrome\n")
+    print()

Python/README.md

@@ -1,40 +1,42 @@
-Add links to your code in Alphabetical Order.
-Format: -[Program name](name of the file)
-
-[Binary Search](binarySearch.py)
-
-[Bubble Sort](bubbleSort.py)
-
-[Dijkstra's Algorithm](Dijkstra's.py)
-
-[Union Find Data Structure](ufds.py)
-
-[Kadane's Algorithm to find Largest Sum contiguous subarray](kadanes_algorithm_largest_sum_contiguous_subarray.py)
-
-[Linear Search](LinearSearch.py) 
-
-[Longest Substring Without Repeating Characters](lengthOfLongestSubstring.py)
-
-[Merge Sort](MergeSort.py)
-
-[Palindrome Check](palindrome_check.py)
-
-[Pascal's Triangle](Pascles_Triangle.py)
-
-[Row_with_minimum_number_of 1's](Row_with_minimum_number_of_1's.py)
-
-[Selection Sort](selectionSort.py)
-
-[Shortest Path in Unweighted Graph](shortest_path_unweighted.py)
-
-[Tower of Hanoi(Recusion)](towerOfHanoi.py)
-
-[Row_with_minimum_number_of 1's](Row_with_minimum_number_of_1's.py)
-
-[Floyd Warshall](Floyd.py)
-
-[Maximum Sum Increasing Subsequence](MaximumSumIncreasingSubsequence.py)
-
-[01Knapsack](01Knapsack.py)
-
-[Insertion Sort](Insertion_Sort.py)
+Add links to your code in Alphabetical Order.
+Format: -[Program name](name of the file)
+
+[Binary Search](binarySearch.py)
+
+[Bubble Sort](bubbleSort.py)
+
+[Dijkstra's Algorithm](Dijkstra's.py)
+
+[Union Find Data Structure](ufds.py)
+
+[Kadane's Algorithm to find Largest Sum contiguous subarray](kadanes_algorithm_largest_sum_contiguous_subarray.py)
+
+[Linear Search](LinearSearch.py)
+
+[Linked List Palindrome](LinkedListPalindromeCheck.py)
+
+[Longest Substring Without Repeating Characters](lengthOfLongestSubstring.py)
+
+[Merge Sort](MergeSort.py)
+
+[Palindrome Check](palindrome_check.py)
+
+[Pascal's Triangle](Pascles_Triangle.py)
+
+[Row_with_minimum_number_of 1's](Row_with_minimum_number_of_1's.py)
+
+[Selection Sort](selectionSort.py)
+
+[Shortest Path in Unweighted Graph](shortest_path_unweighted.py)
+
+[Tower of Hanoi(Recusion)](towerOfHanoi.py)
+
+[Row_with_minimum_number_of 1's](Row_with_minimum_number_of_1's.py)
+
+[Floyd Warshall](Floyd.py)
+
+[Maximum Sum Increasing Subsequence](MaximumSumIncreasingSubsequence.py)
+
+[01Knapsack](01Knapsack.py)
+
+[Insertion Sort](Insertion_Sort.py)