合并两个有序链表

递归有时候解决问题也很清晰有啊

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