链表的升级实战

　　　/**
     * 单链表反转
     */
    public static ListNode reverse(ListNode head){
        ListNode pre = null; //当前节点的前一个节点
        ListNode next = null;//当前节点的下一个节点 
        
        while(head != null) {
            next = head.next; //保存下一个节点
            head.next = pre; //重置next
            pre = head; //保存当前节点
            head = next; //重置当前节点
        }
        return pre;
    }
    
    /**
     * 取中间值
     */
    public static ListNode getMid(ListNode head) {
        ListNode fast = head;
        ListNode slow = head;
        while(fast.next != null && fast.next.next != null) {
            fast = fast.next.next;
            slow = slow.next;
        }
        return slow;
    }
    
    
    /**
     * 拆分链表
     */
    public static ListNode[] getLists(ListNode head) {
        ListNode head1 = null;
        ListNode head2 = null;
        ListNode cur1 = null;
        ListNode cur2 = null;
        int count = 0;
        while(head!=null) {
            if(count%2 == 0) {
                if(cur1 != null) {
                    cur1.next = head;
                    cur1 = cur1.next;
                }else {
                    cur1 = head;
                    head1 = cur1;
                }
            }else {
                if(cur2 != null) {
                    cur2.next = head;
                    cur2 = cur2.next;
                }else {
                    cur2 = head;
                    head2 = cur2;
                }
            }
            head = head.next;
            count++;
        }
        if (cur1 != null && cur2 != null) {
            cur1.next = null;
            cur2.next = null;
        }
        if(cur1 != null){
            cur1.next = null;
        }
        if(cur2 != null) {
            cur2.next = null;
        }
        ListNode[] listNodes = new ListNode[]{head1, head2};
        return listNodes;
    }
    
    /**
     * 归并排序链表
     */
    public static ListNode sortList(ListNode head){
        if(head == null ||head.next == null) {
            return head;
        }
        ListNode mid = getMid(head);
        ListNode right = mid.next;
        mid.next = null;//咬断链表
        ListNode node = mergeTwoListNode(sortList(head), sortList(right));
        return node;
        
    }
    
    /**
     * 合并两个链表
     * @param head1
     * @param head2
     */
    public static ListNode mergeTwoListNode(ListNode head1, ListNode head2) {
        if(head1 == null || head2 == null) {
            return head1 == null? head2 : head1;
        }
        ListNode head = null;
        if(head1.value > head2.value) {
            head = head2;
            head.next = mergeTwoListNode(head1, head2.next);
        }else {
            head = head1;
            head.next = mergeTwoListNode(head1.next, head2);
        }
        return head;
    }
    
    public static void main(String[] args) {
        ListNode head = new ListNode(1);
        ListNode node2 = new ListNode(8);
        ListNode node3 = new ListNode(3);
        ListNode node4 = new ListNode(6);
        ListNode node5 = new ListNode(5);
        ListNode node6 = new ListNode(4);
        ListNode node7 = new ListNode(7);
        ListNode node8 = new ListNode(2);
        ListNode node9 = new ListNode(9);
        
        head.next = node2;
        node2.next = node3;
        node3.next = node4;
        node4.next = node5;
        node5.next = node6;
        node6.next = node7;
        node7.next = node8;
        node8.next = node9;
        Mylist.traverse(head);

        ListNode[] nodes = getLists(head);
        ListNode head1 = nodes[0];
        ListNode head2 = nodes[1];
        
        head2 = reverse(head2);
        
        head = mergeTwoListNode(head1, head2);
    
        Mylist.traverse(head);
        Mylist.traverse(sortList(head));
    }