RadixSort.java

public class RadixSort {

    public static void radixSort(int[] arr) {
        if (arr.length == 0) {
            return;
        }

        // Find the maximum number to know number of digits
        int max = getMax(arr);

        // Do counting sort for every digit
        // exp is 10^i where i is current digit number
        for (int exp = 1; max / exp > 0; exp *= 10) {
            countingSortByDigit(arr, exp);
        }
    }

    // A utility function to get maximum value in arr[]
    private static int getMax(int[] arr) {
        int max = arr[0];
        for (int i = 1; i < arr.length; i++) {
            if (arr[i] > max) {
                max = arr[i];
            }
        }
        return max;
    }

    // A function to do counting sort of arr[] according to
    // the digit represented by exp
    private static void countingSortByDigit(int[] arr, int exp) {
        int n = arr.length;
        int[] output = new int[n]; // output array
        int[] count = new int[10];

        // Initialize count array
        for (int i = 0; i < 10; i++) {
            count[i] = 0;
        }

        // Store count of occurrences in count[]
        for (int i = 0; i < n; i++) {
            int digit = (arr[i] / exp) % 10;
            count[digit]++;
        }

        // Change count[i] so that count[i] now contains
        // actual position of this digit in output[]
        for (int i = 1; i < 10; i++) {
            count[i] += count[i - 1];
        }

        // Build the output array
        for (int i = n - 1; i >= 0; i--) {
            int digit = (arr[i] / exp) % 10;
            output[count[digit] - 1] = arr[i];
            count[digit]--;
        }

        // Copy the output array to arr[]
        for (int i = 0; i < n; i++) {
            arr[i] = output[i];
        }
    }

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

    public static void main(String[] args) {
        int[] arr = {64, 25, 12, 22, 11};

        System.out.println("Original array:");
        printArray(arr);

        radixSort(arr);

        System.out.println("Sorted array:");
        printArray(arr);
    }
}