双向链表的插入，删除，和mergesort。

双向链表的插入，删除，mergesort等。

#include<iostream>
using namespace std;
struct BiListNode
{
	int val;
	BiListNode *pre;
	BiListNode *next;
	BiListNode(int x) :val(x), pre(NULL), next(NULL){}
};
//双向链表的插入
BiListNode *insert(BiListNode *head, int num)
{
	BiListNode *node = new BiListNode(num);
	if (head == NULL) return node;
	BiListNode *p = head;
	while (node->val > p->val && p->next != NULL)
	{
		p = p->next;
	}
	if (node->val <= p->val)
	{
		//插入到头节点的位置
		if (p == head)
		{
			node->next = head;
			head->pre = node;
			head = node;
		}
		//插入到中间位置
		else
		{
			p->pre->next = node;
			node->next = p;
			node->pre = p->pre;
			p->pre = node;
		}
	}
	else //比哪个情况都大
	{
		p->next = node;
		node->pre = p;
		node->next = NULL;
	}
	return head;
}
//双向链表的删除
BiListNode *del(BiListNode *head, int num)
{
	BiListNode *p = head;
	while (p->next != NULL && num != p->val)
		p = p->next;
	if (num == p->val)
	{
		if (p == head)
		{
			head = head->next;
			head->pre = NULL;
			delete p;
		}
		else if (p->next == NULL)
		{
			p->pre->next = NULL;
			delete p;
		}
		else
		{
			p->pre->next = p->next;
			p->next->pre = p->pre;
			delete p;
		}
	}
	else
	{
		cout << "not found" << endl;
	}
	return head;
}
//将两个排序链表合并成为一个有序链表
BiListNode *merge(BiListNode *p1, BiListNode *p2)
{
	if (!p1) return p2;
	if (!p2) return p1;
	BiListNode *head1 = p1, *head2 = p2;
	BiListNode *head;
	if (head1->val < head2->val)
	{
		head = head1;
		BiListNode *p= merge(head1->next, head2);
		head->next = p;
		p->pre = head;
	}
	else
	{
		head = head2;
		BiListNode *p= merge(head1, head2->next);
		head->next = p;
		p->pre = head;
	}
	return head;
}
BiListNode *mergeNonrecur(BiListNode *head1, BiListNode *head2)
{
	if (!head1) return head2;
	if (!head2) return head1;
	BiListNode *head, *res;
	if (head1->val < head2->val)
	{
		head = head1;
		res = head;
		head1 = head1->next;
	}
	else
	{
		head = head2;
		res = head;
		head2 = head2->next;
	}
	while (head1 != NULL && head2 != NULL)
	{
		if (head1->val < head2->val)
		{
			head->next = head1;
			head1->pre = head;
			head1 = head1->next;
			head = head->next;
		}
		else
		{
			head->next = head2;
			head2->pre = head;
			head2 = head2->next;
			head = head->next;
		}
	}
	if (head1)
	{
		head->next = head1;
		head1->pre = head;
	}
	else
	{
		head->next = head2;
		head2->pre = head;
	}
	return res;
}
void print(BiListNode *head)
{
	while (head != NULL)
	{
		cout << head->val << " ";
		head = head->next;
	}
	cout << endl;
}
int main()
{
	BiListNode *head1=NULL, *head2=NULL;
	for (int i = 1; i <= 10; i += 2)
	{
		head1 = insert(head1, i);
	}
	head1 = insert(head1, 6);
	for (int i = 2; i <= 10; i += 2)
	{
		head2 = insert(head2, i);
	}
	head2 = insert(head2,1);
	head1 = del(head1, 3);
	head2 = del(head2, 4);

	BiListNode *head3 = mergeNonrecur(head1, head2);
	print(head3);
	print(head1);
	print(head2);
	system("pause");
	return 0;
}