栈与队列

一.算法
什么是算法。独立存在的一种解决问题的方法和思想
二.算法初接触
枚举法

# 如果 a+b+c=1000 ,且 a^2 + b ^2 = c^2 (a,b,c为自然数)，如何求出所有组合

for a in range(0, 1001):

    for b in range(0, 1001):

        for c in range(0 ,1001):
            if a + b + c == 1000 and a**2 + b**2 == c**2:
                print("a= {},b = {}, c= {}".format(a, b, c))
-------------------------------------
a= 0,b = 500, c= 500
a= 200,b = 375, c= 425
a= 375,b = 200, c= 425
a= 500,b = 0, c= 500

改进，因为已经知道了ab，c就是1000-a-b

# 如果 a+b+c=1000 ,且 a^2 + b ^2 = c^2 (a,b,c为自然数)，如何求出所有组合
import time
f_time = time.time()
for a in range(0, 1001):

    for b in range(0, 1001):

       c = 1000-a-b
       if a**2 + b**2 == c**2:
            print("a= {},b = {}, c= {}".format(a, b, c))


e_time = time.time()
print(e_time-f_time)

二.timeit模块

class timeit.Timeit = (stmt=‘pass’,setup=‘pass’,time=)
–>text1 接受执行的代码，“from main import text1” 说明我执---->行text1的时候需要导包,time1 = Timer(“text1()”, “from main import text1”)

import timeit
from timeit import Timer
#
# list_1 = [1,2]
# list_2 = [23,55]
# list_3 = [i for i in range(2, 44)]
# list_4 = list(range(300))



def text1():
    list_5 = []
    for i in range(10000):
        list_5.append(i)


def text2():
    l = []
    for i in range(10000):
        l += [i]


def text3():
    l = [i for i in range(10000)]


def text4():
    l = list(range(10000))


# text1 接受执行的代码，"from __main__ import text1" 说明我执行text1的时候需要导包
time1 = Timer("text1()", "from __main__ import text1")
print(time1.timeit(1000))

time2 = Timer("text2()", "from __main__ import text2")
print(time2.timeit(1000))

time3 = Timer("text3()", "from __main__ import text3")
print(time3.timeit(1000))

time4 = Timer("text4()", "from __main__ import text4")
print(time4.timeit(1000))

二.栈的python实现

class Stack(object):
    """栈"""
    def __init__(self):
        self.__list = []

    def push(self, item):
        # 添加一个新的元素item到栈顶
        self.__list.append(item)

    def pop(self):
        # 弹出栈顶元素
        return self.__list.pop()

    def peek(self):
        # 返回栈顶元素
        if self.__list:
            return self.__list[-1]
        else:
            return None

    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())
    print()

三.队列的python实现

class Queue(object):
    """队列"""
    def __init__(self):
        self.items = []

    def is_empty(self):
        return self.items == []

    def enqueue(self, item):
        """进队列"""
        self.items.insert(0,item)

    def dequeue(self):
        """出队列"""
        return self.items.pop()

    def size(self):
        """返回大小"""
        return len(self.items)


if __name__ == "__main__":
    q = Queue()
    q.enqueue("hello")
    q.enqueue("world")
    q.enqueue("itcast")
    print (q.size())
    print (q.dequeue())
    print (q.dequeue())
    print (q.dequeue())