[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;

    }

};