列表重新排序——Leetcode系列（八）

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}.

My Answer

/**
 * Definition for singly-linked list.
 * class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode(int x) {
 *         val = x;
 *         next = null;
 *     }
 * }
 */
public class Solution {
    public void reorderList(ListNode head) {
        if(head == null || head.next == null){
            return;
        }
        int total = 0;
        ListNode pivot = head;
        //count the number of nodes in the list
        while(pivot != null){
            pivot = pivot.next;
            total++;
        }
        int splitIndex = total / 2 + total % 2;
        pivot = head;
        int shiftTime = 0;
        //get the middle node in the list
        while(shiftTime < splitIndex){
            pivot = pivot.next;
            shiftTime++;
        }
        ListNode secondHead = pivot;
        pivot = pivot.next;
        secondHead.next = null;
        //reverse the second half of the list
        while(pivot != null){
            ListNode temp = pivot;
            pivot = pivot.next;
            temp.next = secondHead;
            secondHead = temp;
        }
        ListNode firstPivot = head;
        ListNode secondPivot = secondHead;
        //get the node from the first half and second half of the list one by one
        while(secondPivot != null){
            ListNode temp = secondPivot;
            secondPivot = secondPivot.next;
            temp.next = firstPivot.next;
            firstPivot.next = temp;
            firstPivot = temp.next;
        }
        if(firstPivot != null){
            firstPivot.next = null;
        }
    }
}

题目来源：https://oj.leetcode.com/problems/reorder-list/