Question

我尝试从此wikipedia page采用自上而下的合并排序算法并将其转换为C代码，但结果并未产生正确的结果。

这是代码：

#include <stdio.h>

int A[10] = {10, 9, 8, 7, 6, 5, 4, 3, 2, 1};
int B[10]; //sorted array
int n;
void TopDownMergeSort(int A[], int B[],int n);
void TopDownSplitMerge(int B[], int iBegin, int iEnd, int A[]);
void TopDownMerge(int A[], int iBegin, int iMiddle, int iEnd, int B[]);
void CopyArray(int A[], int iBegin, int iEnd, int B[]);

int main()
{
    TopDownMergeSort(A, B, 10);
    for(int i = 0;i < 10;i++) {
        printf("%i ", B[i]);
    }
    printf("\n");
    return 0;
}

// Array A[] has the items to sort; array B[] is a work array.
void TopDownMergeSort(int A[], int B[], int n)
{
    CopyArray(A, 0, n, B);           // duplicate array A[] into B[]
    TopDownSplitMerge(B, 0, n, A);   // sort data from B[] into A[]
}

// Sort the given run of array A[] using array B[] as a source.
// iBegin is inclusive; iEnd is exclusive (A[iEnd] is not in the set).
void TopDownSplitMerge(int B[], int iBegin, int iEnd, int A[])
{
    if(iEnd - iBegin < 2)                       // if run size == 1
        return;                                 //   consider it sorted
    // split the run longer than 1 item into halves
    int iMiddle = (iEnd + iBegin) / 2;              // iMiddle = mid point
    // recursively sort both runs from array A[] into B[]
    TopDownSplitMerge(A, iBegin,  iMiddle, B);  // sort the left  run
    TopDownSplitMerge(A, iMiddle,    iEnd, B);  // sort the right run
    // merge the resulting runs from array B[] into A[]
    TopDownMerge(B, iBegin, iMiddle, iEnd, A);
}
void TopDownMerge(int A[], int iBegin, int iMiddle, int iEnd, int B[])
{
//  Left source half is A[ iBegin:iMiddle-1].
// Right source half is A[iMiddle:iEnd-1 
    // While there are elements in the left or right runs...
    for (int k = iBegin; k < iEnd; k++) {
        // If left run head exists and is <= existing right run head.
        if (i < iMiddle && (j >= iEnd || A[i] <= A[j])) {
            B[k] = A[i];
            i = i + 1;
        } else {
            B[k] = A[j];
            j = j + 1;    
        }
    } 
    printf("Sort result 1: ");
    for(int i = 0; i < 10; i++) {
        printf("%i ", A[i]);
    }
    printf("\n");
}

void CopyArray(int A[], int iBegin, int iEnd, int B[])
{
    for(int k = iBegin; k < iEnd; k++)
        B[k] = A[k];
}

在给定列表上运行此代码会生成结果6, 7, 8, 9, 10, 1, 2, 3, 4, 5，并且通常运行给定列表会生成一个列表，其中包含两个未合并的已排序的半部分。我尽可能忠实地复制算法，但我似乎无法弄清问题出在哪里。

Answer 1

维基百科文章中的代码有一个错误。

在TopDownMergeSort中，行：

TopDownSplitMerge(B, 0, n, A);   // sort data from B[] into A[]

错了。 A[]是来源，B[]是目的地。这是更正的代码行：

TopDownSplitMerge(A, 0, n, B);   // sort data from A[] into B[]

自上而下的合并排序算法产生不匹配的列表

1 个答案: