[LeetCode]Merge Two Sorted Lists

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.

/**
 * 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 *p1=l1;
        ListNode *p2=l2;
        ListNode *head=NULL,*r=NULL;
        while(p1&&p2)
        {
            if(head==NULL)
            {
                if(p1->val<p2->val)
                {
                    head=r=p1;
                    p1=p1->next;                }
                else
                {
                    head=r=p2;
                    p2=p2->next;
                }
            }
            else
            {
                if(p1->val<p2->val)
                {
                    r->next=p1;
                    p1=p1->next;
                }
                else
                {
                    r->next=p2;
                    p2=p2->next;
                }
                r=r->next;
            }
        }
        if(p1) r->next=p1;
        if(p2) r->next=p2;
        return head;
    }
};