Create flash_sort.py

This pull request adds an implementation of the Flash Sort algorithm in `sorts/flash_sort.py`.

**Algorithm overview:**
Flash Sort is a distribution-based sorting algorithm especially efficient for large datasets with elements that are uniformly distributed. Its main idea is to classify elements into buckets (classes) using a linear transformation, rearrange the array in-place using a cycle leader permutation, and finally apply insertion sort within each class for local ordering.

**Implementation details:**
- The number of classes (buckets) is empirically set to `int(0.43 * n)` (where `n` is the length of the array), following recommendations from the original paper and Wikipedia. This balance helps avoid both oversparse and overcrowded buckets.
- The implementation includes detailed comments and uses descriptive variable names for clarity.
- The function returns a new sorted list and does not modify the input array in-place.

**Reference:**
- [Wikipedia: Flashsort](https://en.wikipedia.org/wiki/Flashsort)

**Use cases:**  
Most efficient when data is numeric and uniformly distributed. For other distributions, performance may degrade.

Closes #13203

Author: morgen-code

sorts/flash_sort.py

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+# Flash Sort Algorithm
+# 
+# Flash Sort is a distribution sorting algorithm designed for large arrays with elements
+# that are relatively uniformly distributed. The algorithm can achieve close to O(n) time
+# complexity under favorable conditions.
+#
+# Main steps:
+# 1. Find the minimum and maximum values in the array.
+# 2. Choose the number of classes ("buckets") m. The typical choice is m = int(0.43 * n),
+#    where n is the array length. The constant 0.43 is an empirical value shown by the
+#    original paper and Wikipedia to provide good performance in practice. The goal is
+#    to have enough classes to distribute elements evenly, but not so many that classes
+#    become sparse.
+# 3. Classify each element into one of the m classes using a linear mapping from the
+#    value range to class indices.
+# 4. Compute prefix sums of the class counts to determine the class boundaries.
+# 5. Rearrange (permute) elements in-place so that all elements belonging to the same
+#    class are grouped together. This is performed using a cycle leader algorithm.
+# 6. For each class, perform a final sorting step (usually insertion sort), because
+#    elements within a class are not guaranteed to be sorted.
+#
+# Reference:
+# https://en.wikipedia.org/wiki/Flashsort
+
+def flash_sort(array):
+    """
+    Flash Sort algorithm.
+
+    Flash Sort is a distribution sorting algorithm that achieves linear time complexity O(n)
+    for uniformly distributed data sets using relatively little additional memory.
+    See: https://en.wikipedia.org/wiki/Flashsort
+
+    Args:
+        array (list): List of numeric values to be sorted.
+
+    Returns:
+        list: Sorted list.
+    """
+    n = len(array)
+    if n == 0:
+        return array.copy()
+
+    min_value = min(array)
+    max_value = max(array)
+    if min_value == max_value:
+        return array.copy()
+
+    # Step 2: Choose the number of classes (buckets)
+    # Empirically, 0.43 * n gives good performance; see Wikipedia and original papers.
+    number_of_classes = max(int(0.43 * n), 2)
+    class_boundaries = [0] * number_of_classes
+
+    # Step 3: Classify elements into classes (buckets)
+    class_coefficient = (number_of_classes - 1) / (max_value - min_value)
+    for value in array:
+        class_index = int(class_coefficient * (value - min_value))
+        class_boundaries[class_index] += 1
+
+    # Step 4: Compute prefix sums for class boundaries
+    for i in range(1, number_of_classes):
+        class_boundaries[i] += class_boundaries[i - 1]
+
+    # Step 5: Permute elements into correct classes (cycle leader permutation)
+    sorted_array = [0] * n
+    for value in reversed(array):
+        class_index = int(class_coefficient * (value - min_value))
+        class_boundaries[class_index] -= 1
+        sorted_array[class_boundaries[class_index]] = value
+
+    # Step 6: Final insertion sort within the sorted array
+    for i in range(1, n):
+        key = sorted_array[i]
+        j = i - 1
+        while j >= 0 and sorted_array[j] > key:
+            sorted_array[j + 1] = sorted_array[j]
+            j -= 1
+        sorted_array[j + 1] = key
+
+    return sorted_array