021合并两个链表并排序

//此题所谓的排序，并不是要把合并后的数字排序，而是将l1,l2的数字合并

struct ListNode {
    int val;
    ListNode *next;
    ListNode(int x) : val(x), next(NULL) {}//构造函数
   
};
 

//使用小学生插队思想，多次遍历链表进行排序，不推荐
ListNode* mergeTwoLists(ListNode* l1, ListNode* l2) {
    if (l2 == NULL) return l1;
    if (l1 == NULL) return l2;
    ListNode *p1 = l1;
    ListNode *p2 = l2;
    ListNode *start = l1->val < l2->val ? l1 : l2;
    do {
        if (p2->val <= p1->val) {
            while (p2->next != NULL && p2->next->val <= p1->val) {
                p2 = p2->next;
            }
            l2 = p2->next;
            p2->next = p1;
        }
        else {
            while (p1->next != NULL && p1->next->val < p2->val) {
                p1 = p1->next;
            }
            l2 = p2->next;
            p2->next = p1->next;
            p1->next = p2;
        }
        p2 = l2;
    } while (p2 != NULL);
    return start;


}

//自己答案
ListNode* mergeTwoLists(ListNode* l1, ListNode* l2) {
    ListNode* Res=new ListNode(0), *p = Res;
    if (l2 == NULL) return l1;
    if (l1 == NULL) return l2;
    while (l1 && l2) {
        if (l1->val > l2->val) {
            p->next = l2;
            l2 = l2->next;
        }
        else {
            p->next = l1;
            l1 = l1->next;
        }
        p = p->next;

    }
    p->next = l1 ? l1 : l2;//将比较长的链表数据补充上来
    return Res->next;//去除头结点；

}

//递归思想
ListNode* mergeTwoLists(ListNode* l1, ListNode* l2) {
    if (l2 == NULL) return l1;
    if (l1 == NULL) return l2;
    ListNode*Res = l1->val <= l2->val ? l1 : l2;//取小
    ListNode*p = l1->val > l2->val ? l1 : l2;//取大
    Res->next = mergeTwoLists(Res->next, p);
    return Res;
}

//算法加递归
ListNode* mergeTwoLists(ListNode* l1, ListNode* l2) {
    if (!l1 || (l2 && l1->val > l2->val))swap(l1, l2);//每次将小者给l1
    if (l1)l1->next = mergeTwoLists(l1->next, l2);
    return l1;
    
}


void T21() {
    ListNode *l1,*l2;
    ListNode *p, *q;
    l1 = new ListNode(0);
    l2 = new ListNode(0);


    p = l1;//带头结点
    q = new ListNode(0);
    q->val = 8;
    p->next = q;
    p = q;


    q = new ListNode(0);
    q->val = 3;
    p->next = q;
    p = q;

    q = new ListNode(0);
    q->val = 1;
    p->next = q;
    p = q;
    p->next = NULL;

    p = l2;/**************************/
    q = new ListNode(0);
    q->val = 4;
    p->next = q;
    p = q;
    q = new ListNode(0);
    q->val = 9;
    p->next = q;
    p = q;

    q = new ListNode(0);
    q->val =6;
    p->next = q;
    p = q;

    q = new ListNode(0);
    q->val = 6;
    p->next = q;
    p = q;
    p->next = NULL;
    

    ListNode* Res = l1;
    Res = mergeTwoLists(l1,l2);
    while (Res) {
        cout << Res->val << "\t";
        Res = Res->next;
    }
    cout << endl;

}