Problem21

class Solution {
    //递归，每次比较l1和l2当前的值，取小的一个，直到有一个取空
    public ListNode mergeTwoLists(ListNode l1, ListNode l2) {
        if (l1 == null || l2 == null) return l1 == null ? l2 : l1;
        if(l1.val < l2.val){
            l1.next = mergeTwoLists(l1.next, l2);
            return l1;
        } else{
            l2.next = mergeTwoLists(l1, l2.next);
            return l2;
        }
    }
}