python算法第四课感悟

单向循环链表

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。

实现代码

# coding:utf-8

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

class SingCycleLinst(object):
    """单向循环链表"""
    def __init__(self, node = None):
        self.__head = None
        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
        count = 1
        cur = self.__head
        # count 记录数量

        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指向尾结点，但尾结点元素未打印，需要专门打印一次。


        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指向尾结点，需要将尾结点指向头节点形成单向循环链表。


            node.next = self.__head
            self.__head = node
            cur.next = node

    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
            cur.next = node

    def insert(self, pos, item):
        """指定位置添加元素"""
        if pos <= 0:
            self.add(item)
        elif pos > (self.length()-1):
            self.append()
        else:
            node = Node(item)
            cur = self.__head
            count = 0
            while count < (pos -1):
                count += 1
                cur = cur.next
                # 当循环退出后，cur指向pos-1位置
            node.next = cur.next
            cur.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:
                if cur == self.__head:
                    # 头节点
                    # 找尾结点
                    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指向尾结点
        if cur.elem == item:
            if cur == self.__head:
                self.__head = None
            else:
                pre.next = cur.next

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

if __name__ == "__main__":
    scl = SingCycleLinst()
    print(scl.is_empty())
    print(scl.length())
    print()

    scl.append(1)
    scl.append(5)
    scl.append(45)
    scl.length()
    scl.travel()
    print()

    scl.add(74)
    scl.add(45265)
    scl.travel()

    scl.remove(5)
    scl.travel()

    scl.remove(45265)
    scl.travel()

栈和队列

栈——相当于单口容器——后进先出（LIFO, Last In First Out）

栈结构实现

栈可以用顺序表实现，也可以用链表实现。

以顺序表实现为例

# coding:utf-8

class Stack(object):
    """栈"""

    def __init__(self):
        self.__list = []

    def push(self, item):
        """添加一个新元素item到栈顶"""
        self.__list.append(item)

    def pop(self):
        """d出栈顶元素"""
        return self.__list.pop()

    def peek(self):
        """返回栈顶元素"""
        if self.__list:
            return self.__list[-1]

    def is_empty(self):
        """判断栈是否为空"""
        return self.__list == []

    def size(self):
        """返回栈的元素个数"""
        return len(self.__list)


if __name__ == "__main__":
    s = Stack()
    s.push(1)
    s.push(2)
    s.push(3)
    print(s.pop())
    print(s.pop())
    print(s.pop())

队列——先进先出（FIFO, First In First Out）

队列（queue）是只允许在一端进行插入 *** 作，而在另一端进行删除 *** 作的线性表。

队列结构实现

队列可以用顺序表实现，也可以用链表实现。

以顺序表实现为例

# coding:utf-8

class Queue(object):
    # 队列，先进先出。


    def __init__(self):
        self.__list = []

    def enqueue(self, item):
        """往队列中添加一个item元素"""
        self.__list.append(item)

    def dequeue(self):
        """从队列头部删除一个元素"""
        return self.__list.pop(0)

    def is_empty(self):
        """判断队列是否为空"""
        return self.__list == None

    def size(self):
        """返回队列的大小"""
        return len(self.__list)


if __name__ == "__main__":
    q = Queue()
    print(q.is_empty())
    print(q.size())
    print()

    q.enqueue(1)
    q.enqueue(2)
    q.enqueue(3)
    q.enqueue(4)
    print()

    print(q.is_empty())
    print(q.size())
    print()

    print(q.dequeue())
    print(q.dequeue())
    print(q.dequeue())
    print(q.dequeue())
    print(q.is_empty())

双端队列的实现

# coding:utf-8

class DoubleQueue(object):
    # 双端队列，先进先出。


    def __init__(self):
        self.__list = []

    def add_font(self, item):
        """往队列中添加一个item元素"""
        self.__list.insert(0, item)

    def add_rear(self, item):
        """往队列中添加一个item元素"""
        self.__list.append(item)

    def pop_front(self):
        """从队列头部删除一个元素"""
        return self.__list.pop(0)

    def pop_rear(self):
        """从队列尾部删除一个元素"""
        return self.__list.pop()

    def is_empty(self):
        """判断队列是否为空"""
        return self.__list == None

    def size(self):
        """返回队列的大小"""
        return len(self.__list)


if __name__ == "__main__":
    dq = DoubleQueue()
    print("is_empty?:{0}".format(dq.is_empty()))
    print("Size:{0}".format(dq.size()))
    print()

    dq.add_font(1)
    dq.add_font(2)
    dq.add_font(3)
    dq.add_font(4)
    dq.add_font(5)
    dq.add_font(6)
    dq.add_font(7)
    dq.add_font(8)
    dq.add_font(9)
    dq.add_font(10)
    dq.add_rear(101)
    dq.add_rear(202)
    dq.add_rear(303)
    dq.add_rear(404)
    dq.add_rear(505)
    dq.add_rear(606)
    dq.add_rear(707)
    dq.add_rear(808)
    dq.add_rear(909)
    print()

    print("is_empty?:{0}".format(dq.is_empty()))
    print("Size:{0}".format(dq.size()))
    print()

    print(dq.pop_front(), end=" ")
    print(dq.pop_front(), end=" ")
    print(dq.pop_front(), end=" ")
    print(dq.pop_front(), end=" ")
    print("is_empty?:{0}".format(dq.is_empty()))
    print()

    print(dq.pop_rear(), end=" ")
    print(dq.pop_front(), end=" ")
    print(dq.pop_rear(), end= " ")
    print(dq.pop_rear(), end= " ")
    print(dq.pop_front(), end= " ")
    print(dq.pop_front(), end= " ")
    print(dq.pop_rear(), end= " ")
    print(dq.pop_rear(), end= " ")
    print("Size:{0}".format(dq.size()))
    print("is_empty?:{0}".format(dq.is_empty()))