Solution.java

/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode() {}
 *     ListNode(int val) { this.val = val; }
 *     ListNode(int val, ListNode next) { this.val = val; this.next = next; }
 * }
 */
class Solution {
    public void reorderList(ListNode head) {
        if(head==null||head.next==null){
            return;
        }
        
        ListNode slow = head;
        ListNode fast = head;
        
        while(fast.next!=null&&fast.next.next!=null){
            slow = slow.next;
            fast = fast.next.next;
        }
        
        
        ListNode h2 = reverse(slow.next);
        slow.next = null;
        
        //merge two linkedlist
        
        while(head!=null&&h2!=null){
            ListNode h1tmp = head.next;
            ListNode h2tmp = h2.next;
            h2.next = head.next;
            head.next = h2;
            head = h1tmp;
            h2 = h2tmp;
        }
        
    }
    
    private ListNode reverse(ListNode head){
        ListNode pre = null;
        ListNode cur = head;
        while(cur!=null){
            ListNode tmp = cur.next;
            cur.next = pre;
            pre = cur;
            cur = tmp;
        }
        return pre;
    }
}