Create RadixSort.java

Java/algorithms/sorting/RadixSort.java

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+/*
+File path (suggested): algorithms/sorting/RadixSort.java
+(If you want to use a package, uncomment the package line below and place the file accordingly)
+*/
+// package algorithms.sorting;
+
+import java.util.Arrays;
+
+/**
+ * Algorithm Name : Radix Sort
+ * Programming Language: Java
+ * Category: Sorting
+ * Difficulty Level: Medium (Intermediate)
+ *
+ * Problem Statement:
+ * Implement Radix Sort (LSD - least significant digit first) to sort an array
+ * of non-negative integers. Use Counting Sort as a stable subroutine to sort
+ * based on individual digit positions.
+ *
+ * Requirements:
+ * - Input: array of non-negative integers
+ * - Output: sorted array in ascending order
+ *
+ * Complexities:
+ * - Time Complexity (Worst/Best/Average): O(d * (n + b))
+ *     where d = number of digits in the maximum element,
+ *           n = number of elements,
+ *           b = base (10 for decimal)
+ * - Space Complexity: O(n + b)
+ *
+ * Notes:
+ * - This implementation assumes non-negative integers. To handle negative numbers,
+ *   separate negatives and positives, radix-sort their absolute values, then merge.
+ */
+public class RadixSort {
+
+    /**
+     * Utility to get maximum value in arr[] — used to determine how many digits.
+     */
+    private static int getMax(int[] arr) {
+        if (arr == null || arr.length == 0) return 0;
+        int max = arr[0];
+        for (int v : arr) {
+            if (v > max) max = v;
+        }
+        return max;
+    }
+
+    /**
+     * A stable Counting Sort that sorts arr[] according to the digit represented by exp.
+     * exp = 1  -> sort by 1's digit
+     * exp = 10 -> sort by 10's digit
+     * exp = 100 -> sort by 100's digit, etc.
+     *
+     * Complexity of this subroutine: O(n + b) where b = 10 (digits 0..9)
+     */
+    private static void countingSortByDigit(int[] arr, int exp) {
+        int n = arr.length;
+        if (n == 0) return;
+
+        int[] output = new int[n];   // output array to store sorted order by current digit
+        int[] count = new int[10];   // count array for digits 0..9 (base 10)
+
+        // 1) Count occurrences of each digit in the given exp place
+        for (int i = 0; i < n; i++) {
+            int digit = (arr[i] / exp) % 10;
+            count[digit]++;
+        }
+
+        // 2) Convert count[] so that it contains actual positions (prefix sums)
+        for (int i = 1; i < 10; i++) {
+            count[i] += count[i - 1];
+        }
+
+        // 3) Build the output array from right to left to maintain stability
+        //    (stable because elements with same digit appear in original relative order)
+        for (int i = n - 1; i >= 0; i--) {
+            int digit = (arr[i] / exp) % 10;
+            int pos = count[digit] - 1;   // index in output[]
+            output[pos] = arr[i];
+            count[digit]--;               // decrease count for this digit
+        }
+
+        // 4) Copy the output array back to arr[], so arr[] now contains numbers
+        //    sorted by current digit (exp)
+        System.arraycopy(output, 0, arr, 0, n);
+    }
+
+    /**
+     * Main Radix Sort routine — sorts the array using digit-by-digit counting sort.
+     */
+    public static void radixSort(int[] arr) {
+        if (arr == null || arr.length <= 1) return; // already sorted or nothing to do
+
+        // Find the maximum number to know the number of digits
+        int max = getMax(arr);
+
+        // Do counting sort for every digit. Note that instead of passing the digit number,
+        // exp is passed. exp is 10^i where i is current digit number (0 -> LSD).
+        for (int exp = 1; max / exp > 0; exp *= 10) {
+            countingSortByDigit(arr, exp);
+        }
+    }
+
+    /** Utility to print array in a readable form. */
+    private static void printArray(String label, int[] arr) {
+        System.out.println(label + Arrays.toString(arr));
+    }
+
+    /** Sample test cases covering: small arrays, repeated numbers, single-element array */
+    public static void main(String[] args) {
+        // Test case 1: Small array (classic example)
+        int[] arr1 = {170, 45, 75, 90, 802, 24, 2, 66};
+        printArray("Original arr1: ", arr1);
+        radixSort(arr1);
+        printArray("Sorted   arr1: ", arr1);
+        System.out.println();
+
+        // Test case 2: Array with repeated numbers
+        int[] arr2 = {5, 3, 5, 2, 8, 5, 3};
+        printArray("Original arr2: ", arr2);
+        radixSort(arr2);
+        printArray("Sorted   arr2: ", arr2);
+        System.out.println();
+
+        // Test case 3: Single-element array
+        int[] arr3 = {42};
+        printArray("Original arr3: ", arr3);
+        radixSort(arr3);
+        printArray("Sorted   arr3: ", arr3);
+        System.out.println();
+
+        // Optional: Empty array test
+        int[] arr4 = {};
+        printArray("Original arr4: ", arr4);
+        radixSort(arr4);
+        printArray("Sorted   arr4: ", arr4);
+    }
+}