[数据结构与算法]算法设计——分治算法

2.3二分搜索

def BinarySearch(my_list,x):
    left =0
    right = len(my_list)
    while left<=right:
        middle = (left+right)//2
        if x==my_list[middle]:
            return middle
        elif x>my_list[middle]:
            left = middle+1
        else:
            right = middle-1
    return -1

2.7合并排序

def Merge(a,b):
    c=[]
    i = j =0
    while i <len(a) and j < len(b):
        if a[i]<b[j]:
            c.append(a[i])
            i+=1
        else:
            c.append(b[j])
            j+=1
    if i == len(a):
        c +=b[j:]
    else:
        c +=a[i:]
    return c

def MergeSort(my_list):
    if len(my_list)<=1:
        return my_list
    middle = len(my_list)//2
    left = MergeSort(my_list[:middle])
    right = MergeSort(my_list[middle:])
    return Merge(left,right)


if __name__ == '__main__':
    a = [4, 7, 8, 3, 5, 9]
    print(MergeSort(a))

2.8快速排序

def Partition(mylist,left,right):
    i = left+1
    j = right
    x = mylist[left]
    while True:
        while mylist[i]<=x and i<right:
            i+=1
        while mylist[j]>x and j>left:
            j-=1
        if i>=j:
            break
        mylist[i],mylist[j] = mylist[j],mylist[i]

    mylist[left] = mylist[j]
    mylist[j] = x
    return j

def QuickSort(mylist,left,right):
    if left < right:
        q = Partition(mylist,left,right)
        QuickSort(mylist,left,q-1)
        QuickSort(mylist,q+1,right)

if __name__ == '__main__':
    a = [1,3,56,1,8,2,1,9,4]
    print(QuickSort(a,0,len(a)-1))
    print(a)