Reorder List Program (Leetcode)

Reorder List Program (Leetcode:

You are given the head of a singly linked list. The list can be represented as:

L0 → L1 → … → Ln - 1 → Ln

Reorder the list to be on the following form:

L0 → Ln → L1 → Ln - 1 → L2 → Ln - 2 → …

You may not modify the values in the list's nodes. Only nodes themselves may be changed.

 

Example 1:

Input: head = [1,2,3,4]
Output: [1,4,2,3]

Example 2:

Input: head = [1,2,3,4,5]
Output: [1,5,2,4,3]

 

Constraints:

• The number of nodes in the list is in the range [1, 5 * 104].
• 1 <= Node.val <= 1000

print 'hello world!'# Definition for singly-linked list.
# class ListNode:
#     def __init__(self, val=0, next=None):
#         self.val = val
#         self.next = next
class Solution:
    def reorderList(self, head: ListNode) -> None:
        if not head:
            return
        
        temp = head
        mid = head
        
        while temp != None and temp.next != None:
            mid = mid.next
            temp = temp.next.next
            
        rev = self.reverseList(mid)
        
        new = head
        while rev.next != None:
            temp_new = new.next
            new.next = rev
            
            temp_rev = rev.next
            rev.next = temp_new
            
            new = temp_new
            rev = temp_rev
            
    def reverseList(self, head):
        p1 = None
        p2 = head
        
        while p2 != None:
            p3 = p2.next
            p2.next = p1
            p1 = p2
            p2 = p3
        return p1