Create is_Palindrome (TheAlgorithms#740)

Author: Ishani08

data_structures/linked_list/is_Palindrome

@@ -0,0 +1,77 @@
+def is_palindrome(head):
+ if not head:
+ return True
+ # split the list to two parts
+ fast, slow = head.next, head
+ while fast and fast.next:
+ fast = fast.next.next
+ slow = slow.next
+ second = slow.next
+ slow.next = None # Don't forget here! But forget still works!
+ # reverse the second part
+ node = None
+ while second:
+ nxt = second.next
+ second.next = node
+ node = second
+ second = nxt
+ # compare two parts
+ # second part has the same or one less node
+ while node:
+ if node.val != head.val:
+ return False
+ node = node.next
+ head = head.next
+ return True
+
+
+def is_palindrome_stack(head):
+ if not head or not head.next:
+ return True
+
+ # 1. Get the midpoint (slow)
+ slow = fast = cur = head
+ while fast and fast.next:
+ fast, slow = fast.next.next, slow.next
+
+ # 2. Push the second half into the stack
+ stack = [slow.val]
+ while slow.next:
+ slow = slow.next
+ stack.append(slow.val)
+
+ # 3. Comparison
+ while stack:
+ if stack.pop() != cur.val:
+ return False
+ cur = cur.next
+
+ return True
+
+
+def is_palindrome_dict(head):
+ if not head or not head.next:
+ return True
+ d = {}
+ pos = 0
+ while head:
+ if head.val in d.keys():
+ d[head.val].append(pos)
+ else:
+ d[head.val] = [pos]
+ head = head.next
+ pos += 1
+ checksum = pos - 1
+ middle = 0
+ for v in d.values():
+ if len(v) % 2 != 0:
+ middle += 1
+ else:
+ step = 0
+ for i in range(0, len(v)):
+ if v[i] + v[len(v) - 1 - step] != checksum:
+ return False
+ step += 1
+ if middle > 1:
+ return False
+ return True