leetcode之SortList

public static ListNode sortList(ListNode head) {
        if (head == null || head.next == null)
            return head;
        ListNode p = getMid(head);
        ListNode q;
        q = p.next;
        p.next = null;
        p = head;
        p = sortList(p);
        q = sortList(q);
        return merge(p, q);
    }
    
    public static ListNode getMid(ListNode head) {
        ListNode p = head;
        ListNode q = head;
        while (p.next != null && q.next != null && q.next.next != null) {
            p = p.next;
            q = q.next.next;
        }
        return p;
        
    }
    
    public static ListNode merge(ListNode p, ListNode q) {
        ListNode r = new ListNode(0);
        ListNode res = r;
        while (p != null || q != null) {
            int a,b;
            if (p == null)
                a = Integer.MAX_VALUE;
            else
                a = p.val;
                
            if (q == null)
                b = Integer.MAX_VALUE;
            else
                b = q.val;
                
            if (a