2018-07-06单链表与顺序表的对比+单向循环链表

头插法,没有循环,时间复杂度为O(1)
尾插法,需要从头到尾遍历,时间复杂度为O(n)


链表和顺序表的区别

为什么还要用链表?
链表存数据的时候,如果数据过大,系统无足够大内存,会把整个内存中只要可用的内存串联起来;会多占(分散的)内存
顺序表存储元素的时候可以O(1)迅速定位。
链表和顺序表插入也不一样;插入的时候,链表的时间花费在了遍历上,顺序表花费在了数据搬迁上;所以代表了操作的不同。

单向循环链表

让尾节点指向头节点
定义:单链表的一个变形是单向循环链表,链表中最后一个节点的next域不再为None,而是指向链表的头节点。
代码实现:

# coding:utf-8

class Node(object):
    """节点"""

    def __init__(self, elem):
        self.elem = elem
        self.next = None

class SingleCycleLinkList(object):
    """单向循环链表"""

    def __init__(self, node=None):
        self.__head = node  # 如果是空的直接指向
        # 如果node不为空,要指向回来
        if node:
            node.next = node

    def is_empty(self):
        """链表是否为空"""
        return self.__head == None

    def length(self):
        """链表长度"""
        if self.is_empty():
            return 0
        # cur游标,用来移动遍历节点
        cur = self.__head
        # count记录数量
        count = 1
        while cur.next != self.__head:
            count += 1
            cur = cur.next
        return count

    def travel(self):
        """遍历整个链表"""
        if self.is_empty():
            return
        cur = self.__head
        while cur.next != self.__head:
            print(cur.elem, end=" ")
            cur = cur.next
        # 退出循环,cur指向尾节点,但尾节点的元素未打印
        print(cur.elem)

    def add(self, item):
        """链表头部添加元素,头插法"""
        node = Node(item)
        if self.is_empty():
            self.__head = node
            node.next = node
        else:
            cur = self.__head
            while cur.next != self.__head:
                cur = cur.next
            # 退出循环,cur指向尾节点
            node.next = self.__head
            self.__head = node
            # cur.next = node
            cur.next = self.__head

    def append(self, item):
        """链表尾部添加元素, 尾插法"""
        node = Node(item)
        if self.is_empty():
            self.__head = node
            node.next = node
        else:
            cur = self.__head
            while cur.next != self.__head:
                cur = cur.next
            # node.next = cur.next
            node.next = self.__head
            cur.next = node

    def insert(self, pos, item):
        """指定位置添加元素
        :param  pos 从0开始
        """
        if pos <= 0:
            self.add(item)
        elif pos > (self.length() - 1):
            self.append(item)
        else:
            pre = self.__head
            count = 0
            while count < (pos - 1):
                count += 1
                pre = pre.next
            # 当循环退出后,pre指向pos-1位置
            node = Node(item)
            node.next = pre.next
            pre.next = node

    def remove(self, item):
        """删除节点"""
        if self.is_empty():
            return

        cur = self.__head
        pre = None

        while cur.next != self.__head:
            if cur.elem == item:
                # 先判断此结点是否是头节点
                if cur == self.__head:
                    # 头节点的情况
                    # 找尾节点
                    rear = self.__head
                    while rear.next != self.__head:
                        rear = rear.next
                    self.__head = cur.next
                    rear.next = self.__head
                else:
                    # 中间节点
                    pre.next = cur.next
                return
            else:
                pre = cur
                cur = cur.next
        # 退出循环,cur指向尾节点
        if cur.elem == item:
            if cur == self.__head:
                # 链表只有一个节点
                self.__head = None
            else:
                # pre.next = cur.next
                pre.next = self.__head

    def search(self, item):
        """查找节点是否存在"""
        if self.is_empty():
            return False
        cur = self.__head
        while cur.next != self.__head:
            if cur.elem == item:
                return True
            else:
                cur = cur.next
        # 退出循环,cur指向尾节点
        if cur.elem == item:
            return True
        return False


if __name__ == "__main__":
    ll = SingleCycleLinkList()
    print(ll.is_empty())
    print(ll.length())

    ll.append(1)
    print(ll.is_empty())
    print(ll.length())

    ll.append(2)
    ll.add(8)
    ll.append(3)
    ll.append(4)
    ll.append(5)
    ll.append(6)
    # 8 1 2 3 4 5 6
    ll.insert(-1, 9)  # 9 8 1 23456
    ll.travel()
    ll.insert(3, 100)  # 9 8 1 100 2 3456
    ll.travel()
    ll.insert(10, 200)  # 9 8 1 100 23456 200
    ll.travel()
    ll.remove(100)
    ll.travel()
    ll.remove(9)
    ll.travel()
    ll.remove(200)
    ll.travel()

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