Java implementation LeetCode 143 rearrangements list

143. rearrangement list

Given a single chain L: L0 → L1 → ... → Ln-1 → Ln,
after rearranging changed: L0 → Ln → L1 → Ln -1 → L2 → Ln-2 → ...

You can not just simply change the internal node values, but need to be the actual node exchange.

Example 1:

Given list 1-> 2-> 3-> 4, is rearranged 1-> 4-> 2-> 3.
Example 2:

Given list 1-> 2-> 3-> 4-> 5, rearranged 1-> 5-> 2-> 4-> 3.

/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode(int x) { val = x; }
 * }
 */
class Solution {
     public void reorderList(ListNode head) {
        if (head == null) {
            return;
        }
        ListNode fast = head, slow = head;
        while (fast != null && fast.next != null) {
            fast = fast.next.next;
            slow = slow.next;
        }
        
        ListNode cur = slow.next, pre = null, next = null;
        slow.next = null;
        while (cur != null) {
            next = cur.next;
            cur.next = pre;
            pre = cur;
            cur = next;
        }

        ListNode p1 = head, p2 = pre;
        while (p1 != null && p2 != null) {
            next = p2.next;
            p2.next = p1.next;
            p1.next = p2;
            p1 = p2.next;
            p2 = next;
        }
    }
}