算法 时间复杂度, 空间复杂度, 冒泡排序**, 选择排序, 插入算法, 快速排序**, 希尔算法，计数排序, 二分法查找**

 时间复杂度

 小结:

空间复杂度

算法可视化网站推荐

https://visualgo.net/zh

冒泡排序

 

### 冒泡排序 (************)
### 时间复杂度：最差的情况：O(n^2)  最好的情况：O(n)   空间复杂度:O(1)  并没有开辟新的储存空间
def bubble_sort(li):
    for i in range(len(li)-1):
        flag = True   #用于优化
        for j in range(len(li)-1-i):
            if li[j] > li[j+1]:
                li[j], li[j+1] = li[j+1], li[j]
                flag = False
        if flag:
            return

li = [7,5,4,6,3,8,2,9,1]
bubble_sort(li)
print(li)

选择排序

### 选择排序
### 时间复杂度是：O(n^2)
def select_sort(li):
    for i in range(len(li)):
        minLoc = i
        for j in range(i+1, len(li)):
            if li[minLoc] > li[j]:
                li[minLoc], li[j] = li[j], li[minLoc]

插入算法

### 插入排序
### 时间复杂度是：O(n^2)
def insert_sort(li):

    for i in range(1, len(li)):  ### i=2
        tmp = li[i] ## tmp=li[2]=4
        j = i - 1  ### j = 1 li[1]=7

        while j >= 0 and li[j] > tmp:
            li[j+1] = li[j]  ### [5,7,7,6,3,8,2,9,1]  ==> [5,5,7,6,3,8,2,9,1]
            j = j - 1   ### j = 0  j= -1

        li[j+1] = tmp

优化空间: 应用二分查找来寻找插入点

小结:

快速排序

# 快排
##### 时间复杂度是：O(nlogn)
def partition(li, left, right):  #### O(n)
    tmp = li[left]
    while left < right:
        while left < right and li[right] >= tmp:
            right = right - 1
        li[left] = li[right]
        while left < right and li[left] <= tmp:
            left = left + 1
        li[right] = li[left]

    li[left] = tmp
    return left

def quick_sort(li, left, right):

    if left < right:
        mid = partition(li, left, right)  ### 归位函数

        quick_sort(li, left, mid-1)     #### O(logn)
        quick_sort(li, mid+1, right)

li = [7,5,4,6,3,8,2,9,1]
quick_sort(li,0,len(li)-1)
print(li)

上述4中方法时间比较

import time,random

start = time.time()
li = [random.randint(0,100000) for i in range(10000)]
bubble_sort(li)
print('bubble_sort:',time.time()-start) # 8.805101871490479

start = time.time()
li = [random.randint(0,100000) for i in range(10000)]
select_sort(li)
print('select_sort',time.time()-start)  # 4.129027366638184

start = time.time()
li = [random.randint(0,100000) for i in range(10000)]
insert_sort(li)
print('insert_sort',time.time()-start)  # 3.236048460006714

start = time.time()
li = [random.randint(0,100000) for i in range(10000)]
quick_sort(li,0,len(li)-1)
print('quick_sort',time.time()-start)   # 0.029005050659179688

希尔算法(了解)

 

 

 代码：

 

小结：

 

计数排序(了解)

# 假如有一列数组为
# [7,5,4,6,3,8,2,9,1,9]    对他进行计数
# [0,1,1,1,1,1,1,1,1,2]    代表0有0个,1有1个,2有1个...9有2个  

# 计数排序
def count_sort(li):     # 因为时间复杂度没有系数,所以整体复杂度算O(n)
    count = [0 for _ in range(10)]
    print(count)    # [0, 0, 0, 0, 0, 0, 0, 0, 0, 0]

    for k in li:    # 此循环时间复杂度O(m)
        count[k] += 1
    print(count)    # [0, 1, 1, 1, 1, 1, 1, 1, 1, 2]

    li.clear()
    # 下面循环时间复杂度O(n)
    for k,v in enumerate(count):
        for i in range(v):  # 这层时间复杂度可以忽略不计
            li.append(k)

li = [5,7,4,6,3,8,2,9,1,9]
count_sort(li)
print(li)   # [1, 2, 3, 4, 5, 6, 7, 8, 9, 9]

算法拓展：

动态规划 贪心（分糖果，人民币问题） 背包问题

力扣题：https://leetcode-cn.com/

二分法查找

 

 

##### 二分查找 时间复杂度O(logn)
def bin_search(li, value , low, high):

    if low<=high:
        mid = (low+high) // 2

        if li[mid] == value:
            return mid
        elif li[mid] > value:
            return bin_search(li, value, low, mid-1)
        else:
            return bin_search(li, value, mid+1, high)
    else:
        return

li = [1,2,3,4,5,6,7,8,9]
index = bin_search(li, 3, 0, len(li)-1)
print(index)