python初始条件下的quicksort算法

Question

In the following implementation of the quicksort algorithm in Python:

def quicksort(listT):
    greater=[]
    lower=[]
    pivot=[]
    if len(listT)<=1:
        return listT
    else:
        pivot=listT[0]
        for i in listT:
            if i<pivot:
                lower.append(i)
            elif i>pivot:
                greater.append(i)
            else:
                pivot.append(i)
        lower=quicksort(lower)
        greater=quicksort(greater)
        return lower+pivot+greater

I was wondering what exactly the first condition does in this implementation, for what I see when it divides each part of the list into a greater and lower part, according to the pivot, there would be a moment in which the list has a length lower than 1, but this returned list is not concatenated in any way. Could this condition be changed?

Answer 1

The len(listT)<=1 is needed to terminate the recursion. Quicksort works by dividing the problem into more easily solved subproblems. When the subproblem is an empty list or list of length one, it is already solved (no sorting needed) so the result can be returned directly.

Answer 2

If the initial condition is not stated, then the sort will fail at either

• pivot=listT[0] # because the list may be empty and it will reference an invalid index, or

• lower=quicksort(lower) # actually it will never end because the stack keeps building on this line.