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合并两个排序好的列表，递归算法pass
/**
* Definition for singly-linked list.
* struct ListNode {
* int val;
* ListNode *next;
* ListNode(int x) : val(x), next(NULL) {}
* };
*/
class Solution {
public:
ListNode* mergeTwoLists(ListNode* l1, ListNode* l2) {
if(l1 == nullptr) return l2;
else if(l2 == nullptr) return l1;
ListNode * p;
if(l1->val < l2->val){
p = l1;
p -> next = mergeTwoLists(l1->next,l2);
}
else{
p = l2;
p -> next = mergeTwoLists(l1,l2->next);
}
return p;
}
};