leetcode：两链表合并

我的思想无非就是：比较两链表的某一个值。逐渐向下遍历比较然后加到新链表中

   public ListNode mergeTwoLists(ListNode l1, ListNode l2) {
        ListNode listNode = new ListNode(0);
        ListNode firstNode = listNode;

        while (l1 != null && l2 != null) {
            if (l1.val <= l2.val) {
                listNode.next = l1;
                l1 = l1.next;
            } else {
                listNode.next = l2;
                l2 = l2.next;
            }
            listNode = listNode.next;
        }
        while (l1 != null) {
            listNode.next = l1;
            l1 = l1.next;
            listNode = listNode.next;
        }
        while (l2 != null) {
            listNode.next = l2;
            l2 = l2.next;
            listNode = listNode.next;
        }
        return firstNode.next;
    }

不知道如何进行优化通过查资料发现可以用递归实现通过琢磨研究出递归解法

public ListNode mergeTwoLists(ListNode l1, ListNode l2) {
        if (l1 == null) return l2;
        if (l2 == null) return l1;

        ListNode head = null;
        if (l1.val <= l2.val){
            head = l1;
            head.next = mergeTwoLists(l1.next, l2);
        } else {
            head = l2;
            head.next = mergeTwoLists(l1, l2.next);
        }
        return head;
}