链表经典题Merge Two Sorted Lists

/**
 * 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==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 ;
    }
};