diff --git a/data_structures/linked_list/is_Palindrome b/data_structures/linked_list/is_Palindrome new file mode 100644 index 000000000..acc87c1c2 --- /dev/null +++ b/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