在python中使用归并排序来排列给定的整数列表以获得最小的数字

Question

假设我给定的数组是[9,89,10005,77987]，我在归并排序算法中使用了自定义比较。我想要得到[10005,77987,89,9]，然后用它来排序后得到最小数1000577987899。我使用random.randint(0,100)来生成一个数组。我的输入:[70, 73, 85, 60, 76]我的输出:

before: [70, 73, 85, 60, 76]
['70', '73', '85', '60', '76']
splitting the array ['70', '73', '85', '60', '76']
splitting the array ['70', '73']
merging ['70', '73']
splitting the array ['85', '60', '76']
splitting the array ['60', '76']
merging ['60', '76']
Traceback (most recent call last):
File "venture.py", line 66, in <module>
mergeSort(b)
  File "venture.py", line 33, in mergeSort   
mergeSort(righthalf) #the recursive call after splitting
  File "venture.py", line 41, in mergeSort
if  int(lefthalf[i])*10**len(righthalf[j])+int(righthalf[j]) < int(righthalf[j])*10**len(lefthalf[i])+int(lefthalf[j]): #custom_comparison
IndexError: list index out of range

我代码:

import random
def mergeSort(alist):
    if len(alist)>1:
        print "splitting the array",alist
        mid = len(alist)//2  #the floor div
        lefthalf = alist[:mid]
        righthalf = alist[mid:]
        mergeSort(lefthalf)  #the recursive call after splitting
        mergeSort(righthalf) #the recursive call after splitting
        i=0
        j=0
        k=0

        while i < len(lefthalf) and j < len(righthalf):
            if  int(lefthalf[i])*10**len(righthalf[j])+int(righthalf[j]) < int(righthalf[j])*10**len(lefthalf[i])+int(lefthalf[j]): #custom comparison
                alist[k]=lefthalf[i]
                i=i+1
            else:
                alist[k]=righthalf[j]
                j=j+1
            k=k+1
        while i < len(lefthalf):  #to be used when index j is out of range of len(righthalf)
            alist[k]=lefthalf[i]
            i=i+1
            k=k+1
        while j < len(righthalf): #to be used when index i is out of range of len(lefthalf)
            alist[k]=righthalf[j]
            j=j+1
            k=k+1
        print "merging",alist

a = [random.randint(0,100) for c in range(5)]
print "before:",a
b = map(str, a)
print b
mergeSort(b)
print "new :",b

为什么会出现错误?同样的代码用于常规比较: if lefthalf[i] < righthalf[j]:，我只是将if condition更改为if int(lefthalf[i])*10**len(righthalf[j])+int(righthalf[j]) < int(righthalf[j])*10**len(lefthalf[i])+int(lefthalf[j]): #custom comparison。我哪里错了?

Answer 1

请记住，如果原始列表(alist)是奇数，则righthalf和lefthalf的长度将不同。

while条件是j < len(righthalf), if条件是lefthalf[j]。

当alist是奇数和len(alist) = 2N+1时，在j上升到len(righthalf)-1 = (N+1)-1 = N的情况下，你会得到一个索引超出范围的错误，因为lefthalf[N]超出范围。