Merge two sorted linked lists and return it as a new list. The new list should be made by splicing together the nodes of the first two lists.

Example:

Input: 1->2->4, 1->3->4
Output: 1->1->2->3->4->4

/**
 * 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 || l2==NULL) return (l1==NULL ? l2: l1);
        if(l1->val>l2->val) 
            swap(l1,l2);
        l1->next=mergeTwoLists(l1->next, l2);
        return l1;        
    }//end function
};//end class
 

leetcode#21 Merge Two Sorted Lists