143. Reorder List

题目

Given a singly linked list L: L0→L1→…→Ln-1→Ln,
reorder it to: L0→Ln→L1→Ln-1→L2→Ln-2→…

You must do this in-place without altering the nodes' values.

For example,
Given {1,2,3,4}, reorder it to {1,4,2,3}.

思路

三部曲思路，很直接

• 把list拆成前半段和后半段
• 把后半段reverse
• 把后半段依次插入前半段的相应位置（在148中学到的）

Python

# Definition for singly-linked list.
# class ListNode(object):
#     def __init__(self, x):
#         self.val = x
#         self.next = None

class Solution(object):
    def reorderList(self, head):
        """
        :type head: ListNode
        :rtype: void Do not return anything, modify head in-place instead.
        """
        #3 step methods. 1.find the mid. 2.Reverse the second half. 3.insert one buy one
        if not head or not head.next: return #basecase
        
        #First step, find mid
        slow = fast = head
        while fast and fast.next:
            # pre = slow
            slow = slow.next
            fast = fast.next.next
        shalf = slow.next
        slow.next = None #make head to be the first half
        
        #reverse the second half
        r_shalf = None
        while shalf:
            temp = shalf.next
            shalf.next = r_shalf
            r_shalf = shalf
            shalf = temp
            
        #insert the second half to the first half one buy one.
        node = head
        while r_shalf:
            temp = r_shalf.next
            r_shalf.next = node.next
            node.next = r_shalf
            r_shalf = temp
            node = node.next.next