用C ++快速排序

Question

I am implementing the quick sort algorithm from Cormen's Algorithm book(CLRS), but it always prompt "offset out of range", and I don't know how to fix it. Here's my code.

template<typename Iterator>
void quick_sort(Iterator first, Iterator last)
{
    if (last - first > 1)
    {
        auto pivot = partition(first, last);
        quick_sort(first, pivot);
        quick_sort(pivot + 1, last);
    }
}

template<typename Iterator>
Iterator partition(Iterator first, Iterator last)
{
    auto pivot = last - 1;
    auto less_end = first - 1;
    for (auto iter = first; iter != pivot; ++iter)
    {
        if (*iter <= *pivot)
        {
            std::swap(*++less_end, *iter);
        }
    }
    std::swap(*(less_end + 1), *pivot);
    return less_end + 1;
}

Thanks in advance!

Answer 1

When, in your partition() , first is equal to the begin() of the underlying sequence, then:

    auto less_end = first - 1;

becomes undefined behavior.

This is likely to be your problem. If not, use your debugger to step through your code a line at a time, until the error is encountered, and use your debugger to figure out where and why things go off the rails. That's what a debugger is for.

Answer 2

As Sam Varshavchik has already pointed out in his answer , starting at first-1 is the problem. Simplest solution: just shift the whole thing by one, ie start at first , post- instead pre-increment and so on.....

template<typename Iterator>
Iterator partition(Iterator first, Iterator last)
{
    auto pivot = last - 1;
    auto less_end = first;
    for (auto iter = first; iter != pivot; ++iter)
    {
        if (*iter <= *pivot)
        {
            std::swap(*less_end++, *iter);
        }
    }
    std::swap(*(less_end), *pivot);
    return less_end;
}

see http://rextester.com/JCDUL96865