Merge pull request #226 from Pradeepsingh61/add-python-radix-sort

Add Radix Sort implementation in Python

Author: Pradeepsingh61

Python/algorithms/sorting/radix_sort.py

@@ -0,0 +1,129 @@
+"""
+Radix Sort Algorithm
+Time Complexity: O(d * (n + k)) where d is number of digits, k is range of digits
+Space Complexity: O(n + k)
+Author: Karanjot Singh
+Date: October 2025
+Hacktoberfest 2025
+"""
+
+def counting_sort_for_radix(arr, exp):
+    """
+    Counting sort based on digit position (exp)
+    Used as subroutine in radix sort
+
+    Args:
+        arr: Array to sort
+        exp: Current digit position (1, 10, 100, ...)
+    """
+    n = len(arr)
+    output = [0] * n
+    count = [0] * 10
+
+    # Store count of occurrences
+    for i in range(n):
+        index = arr[i] // exp
+        count[index % 10] += 1
+
+    # Change count[i] to contain actual position in output
+    for i in range(1, 10):
+        count[i] += count[i - 1]
+
+    # Build output array
+    i = n - 1
+    while i >= 0:
+        index = arr[i] // exp
+        output[count[index % 10] - 1] = arr[i]
+        count[index % 10] -= 1
+        i -= 1
+
+    # Copy output array to arr
+    for i in range(n):
+        arr[i] = output[i]
+
+
+def radix_sort(arr):
+    """
+    Radix sort implementation for non-negative integers
+
+    Sorts by processing digits from least significant to most significant
+    Uses counting sort as stable sort for each digit
+
+    Args:
+        arr: List of non-negative integers to sort (modified in place)
+    """
+    if not arr:
+        return
+
+    # Find maximum number to determine number of digits
+    max_num = max(arr)
+
+    # Apply counting sort to sort elements based on place value
+    exp = 1
+    while max_num // exp > 0:
+        counting_sort_for_radix(arr, exp)
+        exp *= 10
+
+
+def radix_sort_copy(arr):
+    """
+    Radix sort that returns sorted copy
+
+    Args:
+        arr: List of non-negative integers to sort
+
+    Returns:
+        Sorted copy of the list
+    """
+    arr_copy = arr.copy()
+    radix_sort(arr_copy)
+    return arr_copy
+
+
+# Test cases
+if __name__ == "__main__":
+    print("=== Radix Sort Algorithm ===\n")
+
+    # Test case 1: Random array
+    test1 = [170, 45, 75, 90, 802, 24, 2, 66]
+    print(f"Original array: {test1}")
+    sorted1 = radix_sort_copy(test1)
+    print(f"Sorted array:   {sorted1}")
+
+    # Test case 2: Already sorted
+    test2 = [1, 2, 3, 4, 5]
+    print(f"\nOriginal array: {test2}")
+    sorted2 = radix_sort_copy(test2)
+    print(f"Sorted array:   {sorted2}")
+
+    # Test case 3: Reverse sorted
+    test3 = [543, 321, 123, 54, 21]
+    print(f"\nOriginal array: {test3}")
+    sorted3 = radix_sort_copy(test3)
+    print(f"Sorted array:   {sorted3}")
+
+    # Test case 4: Array with duplicates
+    test4 = [100, 50, 200, 50, 100, 25]
+    print(f"\nOriginal array: {test4}")
+    sorted4 = radix_sort_copy(test4)
+    print(f"Sorted array:   {sorted4}")
+
+    # Test case 5: Single digit numbers
+    test5 = [9, 3, 7, 1, 5, 2, 8]
+    print(f"\nOriginal array: {test5}")
+    sorted5 = radix_sort_copy(test5)
+    print(f"Sorted array:   {sorted5}")
+
+    # Test case 6: Large numbers
+    test6 = [12345, 987, 54321, 123, 9876, 456]
+    print(f"\nOriginal array: {test6}")
+    sorted6 = radix_sort_copy(test6)
+    print(f"Sorted array:   {sorted6}")
+
+    # Test case 7: Single element
+    test7 = [42]
+    print(f"\nOriginal array: {test7}")
+    sorted7 = radix_sort_copy(test7)
+    print(f"Sorted array:   {sorted7}")
+
+    print("\n✅ All test cases completed!")