基数排序

基数排序(不是基于比较的排序):非负数的十进制数 时间复杂度为O(N)，额外空间负载度O(M)
基本思想:

找出要排序的所有数字中的最大数,看他的位数,把其他没有达到位数的数字前面补零,然后准备0~9的10个桶（队列），先按照个位进行入桶，然后按顺序把他们倒出来，先进桶的先倒出，再按照十位，百位。。。依次进行，直到最高位，最后导出的就是排好序的

核心代码

// only for no-negative value
public static void radixSort(int[] arr) {
    if (arr == null || arr.length < 2) {
        return;
    }
    radixSort(arr, 0, arr.length - 1, maxbits(arr));
}

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

public static void radixSort(int[] arr, int begin, int end, int digit) {
    final int radix = 10;
    int i = 0, j = 0;

    int[] bucket = new int[end - begin + 1];
    for (int d = 1; d <= digit; d++) {
        int[] count = new int[radix];
        for (i = begin; i <= end; i++) {
            j = getDigit(arr[i], d);
            count[j]++;
        }
        for (i = 1; i < radix; i++) {
            count[i] = count[i] + count[i - 1];
        }
        for (i = end; i >= begin; i--) {
            j = getDigit(arr[i], d);
            bucket[count[j] - 1] = arr[i];
            count[j]--;
        }
        for (i = begin, j = 0; i <= end; i++, j++) {
            arr[i] = bucket[j];
        }
    }
}

public static int getDigit(int x, int d) {
    return ((x / ((int) Math.pow(10, d - 1))) % 10);
}

全部代码

package class03;

import java.util.Arrays;

public class Code02_RadixSort {

	// only for no-negative value
	public static void radixSort(int[] arr) {
		if (arr == null || arr.length < 2) {
			return;
		}
		radixSort(arr, 0, arr.length - 1, maxbits(arr));
	}

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

	public static void radixSort(int[] arr, int begin, int end, int digit) {
		final int radix = 10;
		int i = 0, j = 0;

		int[] bucket = new int[end - begin + 1];
		for (int d = 1; d <= digit; d++) {
			int[] count = new int[radix];
			for (i = begin; i <= end; i++) {
				j = getDigit(arr[i], d);
				count[j]++;
			}
			for (i = 1; i < radix; i++) {
				count[i] = count[i] + count[i - 1];
			}
			for (i = end; i >= begin; i--) {
				j = getDigit(arr[i], d);
				bucket[count[j] - 1] = arr[i];
				count[j]--;
			}
			for (i = begin, j = 0; i <= end; i++, j++) {
				arr[i] = bucket[j];
			}
		}
	}

	public static int getDigit(int x, int d) {
		return ((x / ((int) Math.pow(10, d - 1))) % 10);
	}

	// for test
	public static void comparator(int[] arr) {
		Arrays.sort(arr);
	}

	// for test
	public static int[] generateRandomArray(int maxSize, int maxValue) {
		int[] arr = new int[(int) ((maxSize + 1) * Math.random())];
		for (int i = 0; i < arr.length; i++) {
			arr[i] = (int) ((maxValue + 1) * Math.random());
		}
		return arr;
	}

	// for test
	public static int[] copyArray(int[] arr) {
		if (arr == null) {
			return null;
		}
		int[] res = new int[arr.length];
		for (int i = 0; i < arr.length; i++) {
			res[i] = arr[i];
		}
		return res;
	}

	// for test
	public static boolean isEqual(int[] arr1, int[] arr2) {
		if ((arr1 == null && arr2 != null) || (arr1 != null && arr2 == null)) {
			return false;
		}
		if (arr1 == null && arr2 == null) {
			return true;
		}
		if (arr1.length != arr2.length) {
			return false;
		}
		for (int i = 0; i < arr1.length; i++) {
			if (arr1[i] != arr2[i]) {
				return false;
			}
		}
		return true;
	}

	// for test
	public static void printArray(int[] arr) {
		if (arr == null) {
			return;
		}
		for (int i = 0; i < arr.length; i++) {
			System.out.print(arr[i] + " ");
		}
		System.out.println();
	}

	// for test
	public static void main(String[] args) {
		int testTime = 500000;
		int maxSize = 100;
		int maxValue = 100000;
		boolean succeed = true;
		for (int i = 0; i < testTime; i++) {
			int[] arr1 = generateRandomArray(maxSize, maxValue);
			int[] arr2 = copyArray(arr1);
			radixSort(arr1);
			comparator(arr2);
			if (!isEqual(arr1, arr2)) {
				succeed = false;
				printArray(arr1);
				printArray(arr2);
				break;
			}
		}
		System.out.println(succeed ? "Nice!" : "Fucking fucked!");

		int[] arr = generateRandomArray(maxSize, maxValue);
		printArray(arr);
		radixSort(arr);
		printArray(arr);

	}

}