11，双向链表与单向循环列表

双向列表

双链表与单链表相似，但是每个节点都有一个指向前一个节点的部分。
想要用python实现双向链表，只需要对单链表稍作改动。

单链表python代码

class SingleLinkList(object):
    def __init__(self, node=None):
        self.__head = node

    def is_empty(self):
        return self.__head == None

    def length(self):
        cur = self.__head
        count = 0
        while cur != None:
            count += 1
            cur = cur.next
        return count

    def travel(self):
        cur = self.__head
        while cur != None:
            print(cur.elem)
            cur = cur.next

    def add(self, item):
        node = Node(item)
        node.next = self.__head
        self.__head = node

    def append(self, item):
        node = Node(item)
        if self.is_empty():
            self.__head = node
        else:
            cur = self.__head
            while cur.next != None:
                cur = cur.next
            cur.next = node

    def insert(self, pos, item):
        if pos <= 0:
            self.add(item)
        elif pos > (self.length()-1):
            self.append(item)
        else:
            node = Node(item)
            pre = self.__head
            count = 0
            while count < (pos-1):
                pre = pre.next
                count += 1
            node.next = pre.next
            pre.next = node

    def remove(self, item):
        cur = self.__head
        pre = self.__head
        while cur != None:
            if cur.elem == item:
                if pre == None:
                    self.__head = cur.next
                else:
                    pre.next = cur.next
            else:
                pre = cur
                cur = cur.next

    def search(self, item):
        cur = self.__head
        while cur != None:
            if cur.elem == item:
                return True
            else:
                cur = cur.next
        return False

双链表python代码

lass Node(object):
    def __init__(self, item):
        self.elem = item
        self.next = None
        self.prev = None

class DoubleLinkList(object):
    def __init__(self, node=None):
        self.__head = node

    def is_empty(self):
        return self.__head is None

    def length(self):
        cur = self.__head
        count = 0
        while cur != None:
            count += 1
            cur = cur.next
        return count

    def travel(self):
        cur = self.__head
        while cur != None:
            print(cur.elem)
            cur = cur.next

    def add(self, item):
        node = Node(item)
        if self.is_empty():
            self.__head = node
        else:
            node.next = self.__head
            self.__head = node
            node.next.prev = node

    def append(self, item):
        node = Node(item)
        if self.is_empty():
            self.__head = node
        else:
            cur = self.__head
            while cur.next != None:
                cur = cur.next
            cur.next = node
            node.prev = cur

    def insert(self, pos, item):
        if pos <= 0:
            self.add(item)
        elif pos > (self.length()-1):
            self.append(item)
        else:
            cur = self.__head

            count = 0
            while count < pos:
                count += 1
                node = Node(item)
            node.next = cur
            node.prev = cur.prev
            cur.prev  = node

    def remove(self, item):
        cur = self.__head
        pre = None
        while cur != None:
            if cur.elem == item:
                if cur == self.__head:
                    self.__head = cur.next
                    if cur.next:
                        cur.next.prev = None
                else:
                    cur.prev.next = cur.next
                    if cur.next:
                        cur.next.prev = cur.prev
                break
            else:
                cur = cur.next

    def search(self, item):
        cur = self.__head
        while cur != None:
            if cur.elem == item:
                return True
            else:
                cur = cur.next
        return False

单向循环列表

单向循环列表的尾节点不指向None，而是指向头节点。
与双向列表相似，只需在单链表的基础上稍作修改，就可以实现单向循环列表。
代码如下：

class Node(object):
    def __init__(self, item):
        self.elem = item
        self.next = None

class DoubleLinkList(object):
    def __init__(self, node=None):
        self.__head = node
        if node:
            node.next = node

    def is_empty(self):
        return self.__head is None

    def length(self):
        cur = self.__head
        count = 0
        while cur.next != self.__head:
            count += 1
            cur = cur.next
        return count

    def travel(self):
        cur = self.__head
        if self.is_empty():
            return
        while cur.next != self.__head:
            print(cur.elem)
            cur = cur.next
        print(cur.elem)

    def add(self, item):
        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 = self.__head
            self.__head = node
            cur.next = self.__head

    def append(self, item):
        node = Node(item)
        if self.is_empty():
            self.__head = node
        else:
            cur = self.__head
            while cur.next != self.__head:
                cur = cur.next
            node.next = cur.next
            cur.next = node

    def insert(self, pos, item):
        if pos <= 0:
            self.add(item)
        elif pos > (self.length() - 1):
            self.append(item)
        else:
            node = Node(item)
            pre = self.__head
            count = 0
            while count < (pos - 1):
                pre = pre.next
                count += 1
            node.next = pre.next
            pre.next = node

    def remove(self, item):
        cur = self.__head
        pre = None
        while cur.next != self.__head:
            if cur.elem == item:
                if cur == self.__head:
                    rear = self.__head
                    while rear.next != self.__head:
                        rear = rear.next
                    self.__head = cur.next
                    rear.next = self.__head
                else:
                    pre.next = cur.next
                return
            else:
                pre = cur
                cur = cur.next
        if cur.elem == item:
            if cur == self.__head:
                self.__head = None
            else:
                pre.next = self.__head

    def search(self, item):
        if self.is_empty():
            return False
        else:
            cur = self.__head
            while cur.next != self.__head:
                if cur.elem == item:
                    return True
                else:
                    cur = cur.next
            if cur.elem == item:
                return True
            return False