Java排序算法：基数排序

基数排序（Radix Sort）

动图演示地址（​​​https://www.cs.usfca.edu/~galles/visualization/RadixSort.html​​​）

代码结构

package suanfa.paixu;

public class RadixSort implements Sort {

    public static void main(String[] args) {
        Test.test(new RadixSort());
    }

    public int maxbits(int[] arr) {
        int max = Integer.MIN_VALUE;
        for (int i = 0; i < arr.length; i++) {
            max = Math.max(max, arr[i]);
        }
        int num = 0;
        while (max > 9) {
            num++;
            max /= 10;
        }
        return num;
    }

    public void radixSort(int[] arr, int l, int r, int digit) {
        final int radix = 10;
        int[] help = new int[r - l + 1];
        for (int d = 0; d <= digit; d++) {
            int[] count = new int[radix];
            for (int i = l; i <= r; i++) {
                count[getDigit(arr[i], d)]++;
            }
            for (int i = 1; i < radix; i++) {
                count[i] = count[i] + count[i - 1];
            }
            for (int i = r; i >= l; i--) {
                help[--count[getDigit(arr[i], d)]] = arr[i];
            }
            for (int i = l, j = 0; l <= r && j < help.length; i++, j++) {
                arr[i] = help[j];
            }
        }
    }

    public int getDigit(int x, int d) {
        return (x / (int) Math.pow(10, d)) % 10;
    }

    public void sort(int[] arr) {
        if (arr == null || arr.length < 2) {
            return;
        }
        // 对负数进行正数化处理，但是有溢出的风险
        int min = positive(arr);
        radixSort(arr, 0, arr.length - 1, maxbits(arr));
        // 还原
        revert(arr, min);
    }

    private int positive(int[] arr) {
        int min = 0;
        for (int i = 0; i < arr.length; i++) {
            min = Math.min(min, arr[i]);
        }

        if (min != 0) {
            for (int i = 0; i < arr.length; i++) {
                arr[i] -= min;
            }
        }
        return min;
    }

    private void revert(int[] arr, int min) {
        if (min != 0) {
            for (int i = 0; i < arr.length; i++) {
                arr[i] += min;
            }
        }
    }
}