leetcode 143. 重排链表

class Solution {
    
    public void reorderList(ListNode head) {
    
        if (head == null) {
    
            return;
        }
        ListNode mid = middleNode(head);
        ListNode l2 = reverseList(mid);
        mergeList(head, l2);
        
    }
     public void merge(ListNode a,ListNode b){
    
        ListNode l1=a;
        ListNode l2=b;
        while(a!=null&&b!=null){
    
            l1=a.next;
            l2=b.next;
            a.next=b;
            a=l1;
            b.next=a;
            b=l2;
        }
     }
    public void mergeList(ListNode l1, ListNode l2) {
    
        ListNode l1_tmp;
        ListNode l2_tmp;
        while (l1 != null && l2 != null) {
    
            l1_tmp = l1.next;
            l2_tmp = l2.next;

            l1.next = l2;
            l1 = l1_tmp;

            l2.next = l1;
            l2 = l2_tmp;
        }
    }
     public ListNode middleNode(ListNode head) {
    
        ListNode slow = head;
        ListNode fast = head;
        while (fast.next != null && fast.next.next != null) {
    
            slow = slow.next;
            fast = fast.next.next;
        }
        return slow;
    }

    public ListNode reverseList(ListNode head) {
    
        ListNode prev = null;
        ListNode curr = head;
        while (curr != null) {
    
            ListNode nextTemp = curr.next;
            curr.next = prev;
            prev = curr;
            curr = nextTemp;
        }
        return prev;
    }
}