Merge two sorted (ascending) linked lists and return it as a new sorted list. The new sorted list should be made by splicing together the nodes of the two lists and sorted in ascending order.

Example

Given1->3->8->11->15->null,2->null, return1->2->3->8->11->15->null.

/**
 * Definition of ListNode
 * class ListNode {
 * public:
 *     int val;
 *     ListNode *next;
 *     ListNode(int val) {
 *         this->val = val;
 *         this->next = NULL;
 *     }
 * }
 */
class Solution {
public:
    /**
     * @param ListNode l1 is the head of the linked list
     * @param ListNode l2 is the head of the linked list
     * @return: ListNode head of linked list
     */
    ListNode *mergeTwoLists(ListNode *l1, ListNode *l2) {
        // write your code here
        if (l1 == NULL &&  l2 == NULL ) {
            return NULL;
        }

        ListNode* dummy = new ListNode(0);
        //dummy->next = head;
        ListNode* cur = dummy;
        while (l1 != NULL && l2 != NULL) {
            if (l1->val < l2->val) {
                cur->next = l1;
                l1 = l1->next;
            } else {
                cur->next = l2;
                l2 = l2->next;
            }
            cur = cur->next;
        }
        if (l1 != NULL) {
            cur->next = l1;
        } else if (l2 != NULL) {
            cur->next = l2;
        }
        return dummy->next;
    }
};