LeetCode第2题（c语言）

typedef struct ListNode{
    int val;
    struct ListNode *next;
}ListNode;

/*
    迭代 
*/
struct ListNode* mergeTwoLists(struct ListNode* l1, struct ListNode* l2){
    if(l1 == NULL) return l2;
    if(l2 == NULL) return l1; 
    struct ListNode* p1 = l1;
    struct ListNode* p2 = l2;
    struct ListNode* l3,*p3;
    if(l1->val < l2->val)
    {
        l3 = l1;
        l1 = l1->next;
        p3 = l3;
    }
    else
    {
        l3 = l2;
        l2 = l2->next;
        p3 = l2;
    }
    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;
    free(l2);
    return l3;
}

/*
    递归 
*/ 
struct ListNode* mergeTwoLists2(struct ListNode* l1, struct ListNode* l2)
{
    if(l1 == NULL) return l2;
    if(l2 == NULL) return l1;
    if(l1->val <= l2->val)
    {
        l1->next = mergeTwoLists(l1->next,l2);
        return l1;
    }
    else
    {
        l2->next = mergeTwoLists(l1,l2->next);
        return l2;
    }
}