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 = l1->val < l2->val ? l1:l2;
        if(head == l1)
            head->next = mergeTwoLists(l1->next, l2);
        else
            head->next = mergeTwoLists(l1, l2->next);
        return head;
    }
};