RadixSort.java

package Sorting;

/**
 * 1) Digit by digit sorting or place by place.
 * Time Complexity: Worst & Average - O(N * d), Best - O(N), Auxiliary Space: O(N)
 * When all elements have the same number of digits, best case time complexity.
 * It is "Stable Sort" as it does not change the order of the duplicate items.
 * It is also "In Place".
 */
public class RadixSort {

    // Finding the Max Element.
    static int findMax(int[] arr) {
        int mx = Integer.MIN_VALUE;
        for (int num : arr) {
            if (num > mx) mx = num;
        }
        return mx;
    }

    // Count Sort for Radix implementation.
    static void countSort(int[] arr, int place) {
        int n = arr.length;
        int[] output = new int[n];

        // Make frequency array
        int[] count = new int[10];
        for (int num : arr) {
            count[(num / place) % 10]++;
        }

        // Make prefix sum array of count array
        for (int i = 1; i < count.length; i++) {
            count[i] += count[i - 1];
        }

        // Find the index of each element in the original array and put it in output, To Make Stable.
        for (int i = n - 1; i >= 0; i--) {
            int idx = count[(arr[i] / place) % 10] - 1;
            output[idx] = arr[i];
            count[(arr[i] / place) % 10]--;
        }

        // Copy all elements of output to array.
        System.arraycopy(output, 0, arr, 0, n);
    }

    // Radix Sort
    static void radixSort(int[] arr) {
        int max = findMax(arr);
        for (int place = 1; max / place > 0; place *= 10) {
            countSort(arr, place);
        }
    }

    public static void main(String[] args) {
        int[] arr = {43, 453, 626, 894, 0, 3};
        radixSort(arr);

        for (int val : arr) {
            System.out.print(val + " ");
        }
        System.out.println();
    }
}