算法入门

算法

算法

什么是算法

算法（Algorithm）：一个计算过程，解决问题的方法

时间复杂度

print('Hello World')            O(1)

for i in range(n):              O(n)
    print('Hello World')        

for i in range(n):              O(n2)
    for j in range(n):        
        print('Hello World')    

for i in range(n):              O(n3)
    for j in range(n):        
        for k in range(n):       
             print('Hello World')

while n > 1:                    O(logn)
    print(n)    
    n = n // 2

 

二分查找法

def bin_search_rec(data_set, value, low, high):    
    if low <= high:        
        mid = (low + high) // 2        
        if data_set[mid] == value:            
            return mid        
        elif data_set[mid] > value:            
            return bin_search_rec(data_set, value, low, mid - 1)        
        else:            return bin_search_rec(data_set, value, mid + 1, high)    
    else:        
        return

排序算法

冒泡排序法

def bubble_sort_1(li):    
    for i in range(len(li)-1):        
        exchange = False        
        for j in range(len(li)-i-1):            
            if li[j] > li[j+1]:                
                li[j], li[j+1] = li[j+1], li[j]      
                exchange = True        
        if not exchange:            
            return

Alt text

选择排序

def select_sort(li):    
    for i in range(len(li) - 1):        
        min_loc = i        
        for j in range(i+1, len(li)):            
            if li[j] < li[min_loc]:                
                min_loc = j        
        if min_loc != i:            
            li[i], li[min_loc] = li[min_loc], li[i]

Alt text

插入排序

def insert_sort(li):    
    for i in range(1, len(li)):        
        tmp = li[i]        
        j = i - 1        
        while j >= 0 and tmp < li[j]:            
            li[j + 1] = li[j]            
            j = j - 1        
        li[j + 1] = tmp

Alt text

快速排序

def quick_sort(data, left, right):    
    if left < right:        
        mid = partition(data, left, right) 
        quick_sort(data, left, mid - 1)        
        quick_sort(data, mid + 1, right)

def partition(data, left, right):    
    tmp = data[left]    
    while left < right:        
        while left < right and data[right] >= tmp: 
            right -= 1        
        data[left] = data[right]        
        while left < right and data[left] <= tmp:  
            left += 1        
        data[right] = data[left]    
    data[left] = tmp    
    return left

归并排序

def mergesort(li, low, high):    
    if low < high:        
        mid = (low + high) // 2        
        mergesort(li, low, mid)        
        mergesort(li, mid + 1, high)        
        merge(li, low, mid, high)

def merge(li, low, mid, high):    
    i = low    
    j = mid + 1    
    ltmp = []    
    while i <= mid and j <= high:        
        if li[i] <= li[j]:            
            ltmp.append(li[i])            
            i += 1        
        else:            
            ltmp.append(li[j])            
            j += 1    
    while i <= mid:        
        ltmp.append(li[i])        
        i += 1    
    while j <= high:        
        ltmp.append(li[j])        
        j += 1
    li[low:high + 1] = ltmp

希尔排序

Alt text

def shell_sort(li):    
    gap = len(li) // 2    
    while gap > 0:        
        for i in range(gap, len(li)):            
            tmp = li[i]            
            j = i - gap            
            while j >= 0 and tmp < li[j]:                
                li[j + gap] = li[j]                
                j -= gap            
            li[j + gap] = tmp        
        gap /= 2

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计数排序

def count_sort(li, max_num):    
    count = [0 for i in range(max_num + 1)]    
    for num in li:        
        count[num] += 1    
    i = 0    
    for num,m in enumerate(count):        
        for j in range(m):            
            li[i] = num            
            i += 1

排序小结

Alt text

posted @ 2018-11-21 23:06  luck_L  阅读(223)  评论(0)  编辑  收藏  举报