chain rearrangement leetcode-143-

Subject description:

method one:

class Solution(object):
    def reorderList(self, head):
        """
        :type head: ListNode
        :rtype: None Do not return anything, modify head in-place instead.
        """
        p = head 
        l = [] 
        while p: 
            l.append(p) 
            p = p.next 
        n = len(l) 
        for i in range(n//2): 
            l[i].next = l[n-i-1] 
            l[n-i-1].next = l[i+1] 
        if n > 0: 
            l[n//2].next = None

Method Two:

class Solution(object):
    def reorderList(self, head):
        """
        :type head: ListNode
        :rtype: None Do not return anything, modify head in-place instead.
        """
        if not head:
            return head
        dummy = ListNode(0)
        dummy.next = head
        fast,slow = head,head
        while fast and fast.next:
            fast = fast.next.next
            slow = slow.next
            
        cur = slow.next
        slow.next = None
        
        p = None
        while cur:
            next_cur = cur.next
            cur.next = p
            p = cur
            cur = next_cur
        pre = head
        while p:
            tem = pre.next
            pre.next = p
            tem2 = p.next
            p.next = tem
            pre = tem
            p = tem2
        return dummy.next