LeetCode || 21.合并两个有序链表

将两个有序链表合并为一个新的有序链表并返回。新链表是通过拼接给定的两个链表的所有节点组成的。 

示例：

输入：1->2->4, 1->3->4
输出：1->1->2->3->4->4

/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     struct ListNode *next;
 * };
 */


struct ListNode* mergeTwoLists(struct ListNode* l1, struct ListNode* l2){
    struct ListNode* p1 = l1;
    struct ListNode* p2 = l2;
    struct ListNode* p = malloc(sizeof(struct ListNode));
    struct ListNode* q = NULL;
    if(p1 &&p2)
    {
        if(p1->val <= p2->val)
        {
            p = p1;
            p1 = p1->next;
        }
        else
        {
            p = p2;
            p2 = p2->next;
        }
    }
    else if(p1 == NULL)
    {
        return p2;
    }
    else
    {
        return p1;
    }

    q = p; //record head

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

    p->next = p1 ? p1:p2;

    return q;

}