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(struct ListNode *l1, struct ListNode *l2){
        if (l1 == NULL && l2 == NULL) {
            return NULL;
        } else if (l1 == NULL) {
            return l2;
        } else if (l2 == NULL) {
            return l1;
        }

        ListNode* phead;
        ListNode* node;
        ListNode* p1 = l1;
        ListNode* p2 = l2;
        if (p1->val <= p2->val) {
            phead = p1;
            p1 = p1->next;
        } else {
            phead = p2;
            p2 = p2->next;
        }
        node = phead;

        while (p1 != NULL && p2 != NULL) {
            if (p1->val <= p2->val) {
                node->next = p1;
                p1 = p1->next;
            } else {
                node->next = p2;
                p2 = p2->next;
            }
            node = node->next;
        }

        node->next = p1 ? p1 : p2;

        return phead;
    }

};