Question

我已经使用Sedgewick在他的课程中教授的算法在Python中编写了Quicksort的实现。我无法正确排序。代码有什么问题？

def partition(a, lo, hi):
    i = lo
    j = hi
    v = a[lo]
    while(True):
        while(a[i] < v):
            i += 1
            if (i == hi): break

        while(a[j] > v):
            j -= 1
            if (j == lo): break

        if (i >= j): break

        a[i], a[j] = a[j], a[i]

    a[lo], a[j] = a[j], a[lo]
    return j

def sort(a, lo, hi):
    if (hi <= lo):
        return
    q = partition(a, lo, hi)
    sort(a, lo, q-1)
    sort(a, q+1, hi)
    assert isSorted(a, lo, hi)

def quick_sort(a):
    shuffle(a)
    sort(a, 0, len(a)-1)
    assert isSortedArray(a)

Answer 1

在尝试理解您自己的参考实现时，您可能会发现这些参考实现很有用。

返回新列表：

def qsort(array):
    if len(array) < 2:
        return array
    head, *tail = array
    less = qsort([i for i in tail if i < head])
    more = qsort([i for i in tail if i >= head])
    return less + [head] + more

对列表进行排序：

def quicksort(array):
    _quicksort(array, 0, len(array) - 1)

def _quicksort(array, start, stop):
    if stop - start > 0:
        pivot, left, right = array[start], start, stop
        while left <= right:
            while array[left] < pivot:
                left += 1
            while array[right] > pivot:
                right -= 1
            if left <= right:
                array[left], array[right] = array[right], array[left]
                left += 1
                right -= 1
        _quicksort(array, start, right)
        _quicksort(array, left, stop)

从可迭代中生成已排序的项目：

def qsort(sequence):
    iterator = iter(sequence)
    head = next(iterator)
    try:
        tail, more = chain(next(iterator), iterator), []
        yield from qsort(split(head, tail, more))
        yield head
        yield from qsort(more)
    except StopIteration:
        yield head

def chain(head, iterator):
    yield head
    yield from iterator

def split(head, tail, more):
    for item in tail:
        if item < head:
            yield item
        else:
            more.append(item)

无法实现Quicksort

1 个答案: