Implemented MSD radix sort algorithm in-place (#4449)

* Implemented MSD radix sort algorithm inplace

* Fixed formatting

sorts/msd_radix_sort.py

@@ -74,6 +74,86 @@ def _msd_radix_sort(list_of_ints: List[int], bit_position: int) -> List[int]:
     return res
 
 
+def msd_radix_sort_inplace(list_of_ints: List[int]):
+    """
+    Inplace implementation of the MSD radix sort algorithm.
+    Sorts based on the binary representation of the integers.
+    >>> lst = [1, 345, 23, 89, 0, 3]
+    >>> msd_radix_sort_inplace(lst)
+    >>> lst == sorted(lst)
+    True
+    >>> lst = [1, 43, 0, 0, 0, 24, 3, 3]
+    >>> msd_radix_sort_inplace(lst)
+    >>> lst == sorted(lst)
+    True
+    >>> lst = []
+    >>> msd_radix_sort_inplace(lst)
+    >>> lst == []
+    True
+    >>> lst = [-1, 34, 23, 4, -42]
+    >>> msd_radix_sort_inplace(lst)
+    Traceback (most recent call last):
+    ...
+    ValueError: All numbers must be positive
+    """
+
+    length = len(list_of_ints)
+    if not list_of_ints or length == 1:
+        return
+
+    if min(list_of_ints) < 0:
+        raise ValueError("All numbers must be positive")
+
+    most_bits = max(len(bin(x)[2:]) for x in list_of_ints)
+    _msd_radix_sort_inplace(list_of_ints, most_bits, 0, length)
+
+
+def _msd_radix_sort_inplace(
+    list_of_ints: List[int], bit_position: int, begin_index: int, end_index: int
+):
+    """
+    Sort the given list based on the bit at bit_position. Numbers with a
+    0 at that position will be at the start of the list, numbers with a
+    1 at the end.
+    >>> lst = [45, 2, 32, 24, 534, 2932]
+    >>> _msd_radix_sort_inplace(lst, 1, 0, 3)
+    >>> lst == [32, 2, 45, 24, 534, 2932]
+    True
+    >>> lst = [0, 2, 1, 3, 12, 10, 4, 90, 54, 2323, 756]
+    >>> _msd_radix_sort_inplace(lst, 2, 4, 7)
+    >>> lst == [0, 2, 1, 3, 12, 4, 10, 90, 54, 2323, 756]
+    True
+    """
+    if bit_position == 0 or end_index - begin_index <= 1:
+        return
+
+    bit_position -= 1
+
+    i = begin_index
+    j = end_index - 1
+    while i <= j:
+        changed = False
+        if not ((list_of_ints[i] >> bit_position) & 1):
+            # found zero at the beginning
+            i += 1
+            changed = True
+        if (list_of_ints[j] >> bit_position) & 1:
+            # found one at the end
+            j -= 1
+            changed = True
+
+        if changed:
+            continue
+
+        list_of_ints[i], list_of_ints[j] = list_of_ints[j], list_of_ints[i]
+        j -= 1
+        if not j == i:
+            i += 1
+
+    _msd_radix_sort_inplace(list_of_ints, bit_position, begin_index, i)
+    _msd_radix_sort_inplace(list_of_ints, bit_position, i, end_index)
+
+
 if __name__ == "__main__":
     import doctest