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python - 由于变量的存储方式，合并排序实现很困难

问题描述

我有一些 C++ 和 Python 经验，但我肯定一直在努力解决这个问题。我得到了合并排序的概念，但我不明白 Python 如何存储这些值。我专注于分类只是[11, 26, 9]为了将这个怪物分成更小的部分。

更具体地说，在下面的输出中，这行之后会发生什么： 1 1 2 (ijk) before 2nd while 循环？Python 执行哪个循环以到达附加输出的最后一行？我的主要困惑是关于 [11] 最终在 9 到 26 之间。哪个循环确实导致了这种情况？

OUTPUT:
[11, 26, 9] left;
[15, 17, 77] right;
[11, 26, 9, 15, 17, 77] myList after dividing by 2;
[11] left;
[26, 9] right;
[11, 26, 9] myList after dividing by 2;
[26] left;
[9] right;
[26, 9] myList after dividing by 2;
[26] left;
[9] right;
[26, 9] myList at zero;
1 length of right;
1 length of left;
0 0 0 (i j k) before if loop;
[9, 9] myList at else;
[26] left before 2nd while loop;
[9] right before 2nd while loop;
0 1 1  (i j k) before 2nd while loop;
[9, 26] myList 2nd while loop i;
[11] left;
[9, 26] right;
[11, 26, 9] myList at zero;
2 length of right;
1 length of left;
0 0 0 (i j k) before if loop;
[9, 26, 9] myList at else;
[11] left;
[9, 26] right;
[9, 26, 9] myList at zero;
2 length of right;
1 length of left;
0 1 1 (i j k) before if loop;
[9, 26, 9] myList at if;
[11] left before 2nd while loop;
[9, 26] right before 2nd while loop;
1 1 2  (i j k) before 2nd while loop;//WHAT HAPPENS HERE NEXT WITH [11]?
[11] left after 2nd while loop j;
[9, 26] right after 2nd while loop j;
[9, 11, 26] myList 2nd while loop j;

import scipy.io as sio, math as m, numpy as np, sympy as sym, scipy as sp

def mergeSort(myList):
    if len(myList) > 1:
        mid = len(myList) // 2
        left = myList[:mid]
        print(left, "left")
        right = myList[mid:]
        print(right, "right")
        print(myList, "myList after dividing by 2")
        mergeSort(left)
        mergeSort(right)
        i, j, k = 0, 0, 0
     
        while i < len(left) and j < len(right):
            print(left, "left")
            print(right,"right")
            print(myList, "myList at zero")
            print(len(right), "length of right")
            print(len(left), "length of left")
            print(i, j, k, "(i j k) before if loop")
            # ---------------------------------------------------------------------    
            if left[i] < right[j]:
                print(myList, "myList at if")
                myList[k] = left[i]
                i += 1
            else:
                myList[k] = right[j]
                print(myList, "myList at else")
                j += 1
           
            k += 1
            # -------------------------------------------------------------------------
        print(left, "left before 2nd while loop")
        print(right, "right before 2nd while loop")
        print(i, j, k, " (i j k) before 2nd while loop")
        # ------------------------------------------------------------------------
        while i < len(left):
            myList[k] = left[i]
            i += 1
            k += 1
            print(myList, "myList 2nd while loop i")
            
        while j < len(right):
            myList[k] = right[j]
            j += 1
            k += 1
            print(left, "left after 2nd while loop j")
            print(right, "right after 2nd while loop j")
            print(myList, "myList 2nd while loop j")
            
myList = [11,26,9,15,17,77]
mergeSort(myList)
print(myList)

标签： pythonarrayssortingmergemergesort

解决方案

我不确定我是否完全理解你的问题，所以如果我的回答不能满足你的问题，请纠正我。

当您调用 mergeSort() 时，长度为 3，因此输入分为左 [11] 和右 [26, 9]。此拆分不会以任何方式影响输入 myList [11, 26, 9]。

在mergesort内部调用之后left仍然是[11]，但是right变成了[9, 26]。

然后第一个 while 循环用 9 覆盖 myList 中的第一个值。在第二次迭代中，它用 11 覆盖 myList 的第二个值。left 和 right 都不受此影响，因此它们仍然保持 left [11] 和 right [9, 26]。(i, j, k) 现在是 (1, 1, 2)，所以第一个 while 循环结束。

第二个while循环被跳过，所以那里没有变化。

在第三个循环中，myList 的剩余第三个值被右侧第二个位置的值 26 覆盖。

这结束了 mySort(...) 的原始调用，并且在此调用之外可以使用修改后的列表。

这是否满足您的问题？

PS：我知道这不是您的目标，但由于 python 不是一种性能良好的语言，请尝试使用内置函数或库，因为它们中的大多数都是用 C/C++ 编译的，因此比自行实现的版本快得多： )

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