leetcode:合并两个有序链表

 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     struct ListNode *next;
 * };
 */
struct ListNode* (struct ListNode* l1, struct ListNode* l2) {
    if (!l2)
        return l1;
    if (!l1)
        return l2;
    
    struct ListNode *p1,*p2=l2,*p3,*l3=NULL,*head1,*head2;
    head1=(struct ListNode*)malloc(sizeof(struct ListNode));
    head2=(struct ListNode*)malloc(sizeof(struct ListNode));
    
    head1->next=l1;head2->next=l2;
    l3=p3=head1;
    while(p1&&p2)
    {
        if(p1->val<=p2->val)
        {
            p3->next=p1;p3=p1;p1=p1->next;
        }
        else
        {
            p3->next=p2;p3=p2;p2=p2->next;
        }
    }
    p3->next=p1?p1:p2;
    
    return l3->next;   
}