Question

我正在尝试实现快速排序算法但不知何故我有一个错误而我无法伪造它，我的randomizedPartition似乎工作正常。这是代码。

#include <iostream>
#include <cstdlib>
#include <cmath>

using namespace std;

int  randomizedPartition(int *v, int start, int end);
void quickSort(int *v, int start, int end);
void swap(int *p, int *q);
void print(int *v, int len);

int main(int argc, char const *argv[]) {

  cout << "QUICK SORT:\n" << endl;
  int v[]      = {6, 4, 7, 3, 0, 1 , 2, 9, 19};
  int len = sizeof(v)/sizeof(int);

  cout << "Before Sorting: " << endl;
  print(v, len);
  cout << "After Sorting: " << endl;
  quickSort(v, 0, len - 1);
  print(v, len);

  return 0;
}

int  randomizedPartition(int *v, int start, int end) {
    int len = (int) abs(end - start + 1);
    srand (time(NULL));
    int idx      = rand() % len;
    cout << "IDX RD: " << idx << endl;
    int pivot    = v[idx];
    swap(&v[start], &v[idx]);

    int i = start - 1;
    int j = start;

    for (int posi = start + 1; posi <= end; ++posi) {
      if(v[posi] <= pivot) {
        ++i;
        ++j;
        swap(&v[posi], &v[i]);
        swap(&v[posi], &v[j]);
      }
    }

    cout << "Pivot: " << pivot << endl;
    cout << "Pivot Index: " << (i + 1) << endl;
    return ++i;
}

void quickSort(int *v, int start, int end) {
    if (start < end) {
      int idx = randomizedPartition(v, start, end);
      print(v, end - start + 1);
      cout << "Start: " << start << endl;
      cout << "End: " << end << endl;
      cout << "idx - 1: " << idx - 1 << endl;
      cout << "idx + 1: " << idx + 1 << endl;
      quickSort(v, start, idx - 1);
      quickSort(v, idx + 1, end);
    }
}

void print(int *v, int len) {
  for (int i = 0; i < len; ++i) {
    cout << *(v + i) << " ";
  }
  cout << "\n" << endl;
}

void swap(int *p, int *q) {
  int temp = *p;
  *p = *q;
  *q = temp;
}

任何人都可以帮助我找出错误吗？以下是可能的输出之一：

Before Sorting: 
6 4 7 3 0 1 2 9 19 

After Sorting: 
IDX RD: 2
Pivot: 7
Pivot Index: 6
4 6 3 0 1 2 7 9 19 

Start: 0
End: 8
idx - 1: 5
idx + 1: 7
IDX RD: 5
Pivot: 2
Pivot Index: 2
0 1 2 6 3 4 

Start: 0
End: 5
idx - 1: 1
idx + 1: 3
IDX RD: 1
Pivot: 1
Pivot Index: 1
0 1 

Start: 0
End: 1
idx - 1: 0
idx + 1: 2
IDX RD: 2
Pivot: 2
Pivot Index: 3
0 1 6 

Start: 3
End: 5
idx - 1: 2
idx + 1: 4
IDX RD: 1
Pivot: 1
Pivot Index: 4
0 3 

Start: 4
End: 5
idx - 1: 3
idx + 1: 5
IDX RD: 1
Pivot: 3
Pivot Index: 7
0 9 

Start: 7
End: 8
idx - 1: 6
idx + 1: 8
0 9 6 2 1 4 7 3 19

Answer 1

randomizedPartition中的索引应为start + rand() % len，而不仅仅是rand() % len。

另外，作为旁注，您的print功能实际上并没有按照您的意愿行事。您希望它通过start打印元素end，但它实际上是通过0打印end - start。一般来说，您的调试过程和编码过程都会受益于尝试保持简洁。现在代码有点乱，这使得调试变得更加困难。

C ++随机快速排序错误

1 个答案: