c++ - 快速选择算法

Question

我正在尝试在具有随机生成数字的数组上实现快速选择算法。现在在用代码编写算法后，它不会从最低到最高对数组进行排序，也无法找到第 k 个最小的元素。我会很感激我得到的帮助。谢谢。

#include<iostream>
#include<ctime>
#include<cstdlib>
using namespace std;

void printArray(int *myArray, int n){
    for(int i = 0; i < n; i++){
       cout << myArray[i] << " ";
}
}

int Partition(int *myArray, int startingIndex, int endingIndex){
int pivot = myArray[endingIndex];
int partitionIndex = startingIndex;
for(int i = startingIndex; i<endingIndex; i++){
    if(myArray[i]<= pivot){
        swap(myArray[i],myArray[partitionIndex]);
        partitionIndex++;
    }
}
swap(myArray[partitionIndex],myArray[endingIndex]);
return partitionIndex;
} 

int QuickSelect(int *myArray, int startingIndex, int endingIndex, int KthElement){
/*if(startingIndex < endingIndex){
    int partitionIndex = Partition(myArray, startingIndex,endingIndex);
    QuickSelect(myArray,startingIndex,partitionIndex-1);
    QuickSelect(myArray,partitionIndex+1,endingIndex);
}*/1
if (startingIndex < endingIndex){
    int partitionIndex = Partition(myArray, startingIndex, endingIndex);
    if(KthElement == partitionIndex)
        return KthElement;
    if(KthElement < partitionIndex)
        QuickSelect(myArray, startingIndex, partitionIndex - 1, KthElement);
    else
        QuickSelect(myArray, partitionIndex + 1, endingIndex, KthElement);
}
}
int main(){

int numOfElements;
int KthElement;
srand(time(NULL));
cout<<"Enter The Amount Of Numbers You Wish To Use: ";
cin >> numOfElements;
int myArray[numOfElements];

cout << "Array Before Sorting: ";
for(int i = 0; i< numOfElements; i++){
    myArray[i] = rand() %10;
}

printArray(myArray, numOfElements);

cout << endl;
cout << endl;

cout <<"Enter The Index Of The Kth Element You Wish To Retrieve: ";
cin >> KthElement;

QuickSelect(myArray, 0,numOfElements,KthElement);
cout << "Array After Sorting: ";
printArray(myArray, numOfElements);
cout << endl;

cout <<"The " <<KthElement<<" Smallest Element Is: " <<   QuickSelect(myArray,0,numOfElements,KthElement);

}

Answer 1

对于 numOfElements作为 5，数组从 0 扩展到 4。你的endingIndex假定它是数组的最后一个索引。

修复:

QuickSelect(myArray, 0,numOfElements-1,KthElement);

你的代码有问题:

• 您的程序访问了

  中的边界数组位置

  int pivot = myArray[endingIndex];
  

• 检查 k<1和 k>(num_of_elements) .

• 同时检查您的代码是否有 num_of_elements = 1。

• 检查 k 对数组的意义，即 For k=1 , arr[0]不应该退回 arr[1] ;