143. Reorder List

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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}.

 

分3步：

1.快慢指针找到中点（1-2-3-4-5-6）

2 反转后半部分（1-2-3-6-5-4）

3.merge （1-6-2-5-4-3-4）

 

 

class Solution {
    public void reorderList(ListNode head) {
        if(head !=null){
            ListNode slower = head;
            ListNode faster = head;
            while(faster != null && faster.next != null){
                faster = faster.next.next;
                slower = slower.next;
            }
            ListNode p2 = reverseList(slower);
            ListNode p1 = head;
            while(p2.next!=null){
                ListNode p1_next  = p1.next;
                ListNode p2_next =  p2.next;
                p2.next = p1.next;
                p1.next = p2;
                p2 = p2_next;
                p1 =p1_next;
            }
        }
   
        
    }
      public ListNode reverseList(ListNode head) {
        if(head == null ||head.next == null) return head;
        ListNode pre = head;
        head = head.next;
        pre.next = null;
        while(head != null){
            ListNode next = head.next;
            head.next = pre;
            pre = head;
            head = next;
        }
        return pre;
    }
}