Program work 16. Radix Sort in Java

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不是基于比较的排序

是基于位数上的排序, 且仅适用于整数.

此位数的排序不会影响前一位数排序好的相对位置, 是stable的排序

影响效能的关键在于数组中最大数字的位数

最差时间复杂度为O(d*n).

另外需要O(n)space

d是最大数字的位数, 决定循环的轮数, n是每轮处理的数目的大小.

Pseudocode

radixSort (arr, tmp){    get the max of the array    d ←the digit number of max    for (i ← digit; i > 0; i--) {        resetCounter        for each same digit position {            counter[pos]++;        }        combineCounter;        for j ← end; j >= 0; j-- {            put the arr[j] to the tmp, pos is determined by the counter;            counter[pos]--;        }}

Java

package ncku.cdc.sorting;import java.util.Random;public class RadixSort {  private int[] sequence;  private int[] counter;  public RadixSort(int size, int range) {    sequence = new int[size];    counter = new int[10];    Random rand = new Random();    for (int i = 0; i < size; i++) {      sequence[i] = rand.nextInt(range);    }  }  public static void main(String[] args) {    int size = Integer.valueOf(args[0]);    int range = Integer.valueOf(args[1]);    RadixSort radix = new RadixSort(size, range);    System.out.println("before radixSort:");    SortingTools.validation(radix.getSequence(), 0);    int[] tmp = new int [size];    radix.radixSort(radix.getSequence(), tmp, size);    System.out.println("after radixSort:");    SortingTools.validation(radix.getSequence(), 0);  }  public void radixSort(int[] arr, int[] tmp, int len) {    int digit = getDigit(arr, len);    int radix = 1;    for (int d = digit; d > 0; d--) {      resetCounter();      int index;      for (int i = 0; i < len; i++) {        index = (arr[i] / radix) % 10;        counter[index]++;      }      caculateCounter();      for (int j = len - 1; j >= 0; j--) {        index = (arr[j] / radix) % 10;        tmp[--counter[index]] = arr[j];      }      radix *= 10;      System.arraycopy(tmp, 0, arr, 0, len);    }  }  private int getDigit(int[] arr, int len) {    int max = Integer.MIN_VALUE;    for (int i = 0; i < len; i++) {      if (arr[i] > max) {        max = arr[i];      }    }    int d = 1;    while ((max /= 10) != 0) {      d++;    }    return d;  }  private void resetCounter() {    for (int i = 0; i < 10; i++) {      counter[i] = 0;    }  }    private void caculateCounter() {    for (int i = 1; i < 10; i++) {      counter[i] += counter[i - 1];    }  }  public int[] getSequence() {    return sequence;  }}

程序输入: 35 200. 表示生成一个长度35, 数字范围为[0, 200)的数组

程序输出:

before radixSort:
0 67 2 173 160 57 36 16 39 76 141 103 128 195 23 71 43 47 160 114 186 6 17 36 158 74 79 133 99 160 28 113 108 171 171
after radixSort:
0 2 6 16 17 23 28 36 36 39 43 47 57 67 71 74 76 79 99 103 108 113 114 128 133 141 158 160 160 160 171 171 173 186 195

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