计数排序-counting sort
概述
在前面的文章中分别给出了插入排序、归并排序、堆排序、快速排序的是实现，我们知道这些排序算法都基于元素之间的比较运算。本文给出计数排序算法，它不是基于元素之间的比较运算，或者说完全没有任意两个元素之间的比较。
代码实现
#include <iostream>
using namespace std;
/**
* It is used to get the maximum for the array.
* @arr store the value of the array
* @length store the array length
* @author Junpeng Zhu
*/
int maximum(int arr[], int length){
int max = arr[0]; // set the arr[0], it is the first element for the array, as the maximum value
for (int i = 1; i < length; i++){
if(arr[i] > max){
max = arr[i];
}else{
continue;
}
}
return max;
}
/**
* counging sort.
* Note: The value of different index for the array is from 0 to k, which k is the maximum.
* @arr store the value of unsorted array
* @length store the length of unsorted array
* @return return sorted array
* @author Junpeng Zhu
*/
int* countingsort(int arr[], const int length){
int max = maximum(arr,length);
int countingLength = max+1;
int* sortedArray = new int[length]; //store the sorted array. Because the length is variable, sortedArray should be allcated in heap area.
int countingArray[countingLength]; //Initial the counting array and the value of elements is 0;
for (int i = 0 ; i < countingLength; i++){
countingArray[i] = 0;
}
for (int i = 0 ; i < length; i++){
countingArray[arr[i]] += 1;
}
for (int i = 1; i < countingLength; i++){
countingArray[i] = countingArray[i] + countingArray[i-1];
}
//put arr into countingArray
for (int i = length-1; i >=0; i--){
sortedArray[countingArray[arr[i]]-1] = arr[i];
countingArray[arr[i]] -= 1;
}
// for(int i = 0 ;i < length;i++){
// cout << sortedArray[i] << " ";
// }
return sortedArray; // resulting hanging pointer. It is not correct.
}
int main()
{
//int arr[] = {3,2,1}; //success
//int arr[] = {2,8,7,1,3,5,6,4}; //success
//int arr[] = {3,5,4,6,2,1}; //success
//int arr[] = {3,5,4,6,2,1}; //success
//int arr[] = {2,5,2,4,1,1}; //success
//int arr[] = {13,19,9,5,12,8,7,4,11,2,6,21}; //success
//cout << length << endl;
int arr[] = {8,10,7,16,10,3,9,7,20,2}; //success
//int arr[] = {8,10,7,16,8,3,9,7,20,2}; //success, compare to the above example {8,10,7,16,10,3,9,7,20,2}
//cout << "arr size:" << sizeof(arr)/sizeof(arr[0]) << endl;
//int arr[] = {1,1,3}; //success
//int arr[] = {1,1,1,1,1,1,1};
int length = sizeof(arr)/sizeof(arr[0]);
//int max = maximum(arr,length);
int *sortedResults = countingsort(arr,length);
for(int i = 0 ;i < length;i++){
cout << sortedResults[i] << " ";
}
return 0;
}