21. 合并两个有序链表

    public ListNode mergeTwoLists(ListNode list1, ListNode list2) {
        ListNode head = new ListNode();
        while (list1 != null) {
            while (list2 != null) {
                ListNode temp = head;
                int val1 = list1.val;
                int val2 = list2.val;
                int maxVal = Math.max(list1.val, list2.val);          
                temp.next = new ListNode(maxVal);      // 较大者在后面
                temp = temp.next; // 指针向前移动
                if (val1 > val2) {
                    list2 = list2.next;
                } else {
                    break;
                }
            }
            list1 = list1.next;
        }
        return head;
    }