LeetCode-Reorder List

Given a singly linked list L: L₀→L₁→…→L_n-1→L_n,
reorder it to: L₀→L_n→L₁→L_n-1→L₂→L_n-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}.

Solution:

/**
 * 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;
        ListNode preHead = new ListNode(0);
        preHead.next = head;
        //Find the median node.
        ListNode p1 = preHead, p2 = preHead;
        while (p2!=null && p2.next!=null){
            p1 = p1.next;
            p2 = p2.next.next;
        }
        
        //Reverse the list after p1.
        p2 = p1.next;
        while (p2.next!=null){
            ListNode temp = p2.next.next;
            p2.next.next = p1.next;
            p1.next = p2.next;
            p2.next = temp;
        }

        //Insert the list after p1 into the list before p1.
        p2 = p1.next;
        p1.next = null;
        p1 = head;
        while (p1!=null && p2!=null){
            ListNode temp = p2.next;
            p2.next = p1.next;
            p1.next = p2;
            p1 = p1.next.next;
            p2 = temp;
        }
        
        
    }
}