就地快速排序实施

Question

我正在尝试按照http://en.wikipedia.org/wiki/Quicksort中的说明实施就地快速排序

下面是python代码，分区功能无法正常工作。

def swap(array, index1, index2):
    tmp = array[index1]
    array[index1] = array[index2]
    array[index2] = tmp

def partition(array, left, right, pivotIndex):
    pivotValue = array[pivotIndex]
    swap(array, pivotIndex, right)
    storeIndex = left
    for i in range(left, right - 1):
        if array[i] < pivotValue:
            swap(array, i, storeIndex)
            storeIndex = storeIndex + 1
            print array, i
    swap(array, storeIndex, right)
    return storeIndex

def quicksort(array, left ,right):
    if right > left:
        print left, right
        pivotIndex = left
        pivotNewIndex = partition(array, left, right, pivotIndex)
        quicksort(array, left, pivotNewIndex - 1)
        quicksort(array, pivotNewIndex + 1, right)

if __name__ == '__main__':
    array = [3,7,8,5,2,1,9,5,4]
    partition(array, 0, len(array) - 1, 3) # 5 is pivot
    print array # expecting all the elements to the left of pivot value(5) will be lesser or equal.

Answer 1

您需要至少进行2个修复：

def partition(array, left, right, pivotIndex):
    pivotValue = array[pivotIndex]
    swap(array, pivotIndex, right)
    storeIndex = left
    for i in range(left, right):    # range doesn't include right element already
        if array[i] <= pivotValue:  # need to check for equality (not really necessary for the sorting routine)
            swap(array, i, storeIndex)
            storeIndex = storeIndex + 1
            print array, i
    swap(array, storeIndex, right)
    return storeIndex

range(left, right)返回类似[left, left + 1, ..., right - 1]的项的列表，因此无需生成带有range(left, right -1)的列表，因为因此，我们不仅将跳过列表的最后一个元素（枢纽所在的位置），还将跳过最后一个元素（即right - 2 ）。

如果期望在partition之后，透视点左边的元素应该小于或等于 ，我们应该在遍历array[i] <= pivotValue过程中将其反映在比较中（ array[i] <= pivotValue ）。

Answer 2

使用更少的变量和干净的方法不是更好的方法吗

＃！/ usr / bin / python

Array = [1,2,3,4,5,4,3,23,4,5,4,3,2,1,2,3,4,3,4,1,412,2,4]

def swap(a,i,j):
    temp=a[i]
    a[i]=a[j]
    a[j]=temp

def partition(a, left, right):

    pivotIndex=right
    for i in range(left,right):
        if a[i] > a[pivotIndex]:
            swap(a,i,pivotIndex)


    return pivotIndex

def quicksort(array , left,right):

    if left<right:
        pivotIndex=partition(array,left,right)
        quicksort(array, left, pivotIndex-1)
        quicksort(array, pivotIndex+1, right)

def main():
    quicksort(Array, 0 , len(Array) -1)   
    print (Array )

main()