More elegant coding for merge_sort_fastest (TheAlgorithms#804)

* More elegant coding for merge_sort_fastest

* More elegant coding for merge_sort

Author: liuzhen153

sorts/merge_sort.py

@@ -29,36 +29,20 @@ def merge_sort(collection):
     >>> merge_sort([-2, -5, -45])
     [-45, -5, -2]
     """
-    length = len(collection)
-    if length > 1:
-        midpoint = length // 2
-        left_half = merge_sort(collection[:midpoint])
-        right_half = merge_sort(collection[midpoint:])
-        i = 0
-        j = 0
-        k = 0
-        left_length = len(left_half)
-        right_length = len(right_half)
-        while i < left_length and j < right_length:
-            if left_half[i] < right_half[j]:
-                collection[k] = left_half[i]
-                i += 1
-            else:
-                collection[k] = right_half[j]
-                j += 1
-            k += 1
-
-        while i < left_length:
-            collection[k] = left_half[i]
-            i += 1
-            k += 1
-
-        while j < right_length:
-            collection[k] = right_half[j]
-            j += 1
-            k += 1
-
-    return collection
+    def merge(left, right):
+        '''merge left and right
+        :param left: left collection
+        :param right: right collection
+        :return: merge result
+        '''
+        result = []
+        while left and right:
+            result.append(left.pop(0) if left[0] <= right[0] else right.pop(0))
+        return result + left + right
+    if len(collection) <= 1:
+        return collection
+    mid = len(collection) // 2
+    return merge(merge_sort(collection[:mid]), merge_sort(collection[mid:]))
 
 
 if __name__ == '__main__':
@@ -69,4 +53,4 @@ def merge_sort(collection):
 
     user_input = raw_input('Enter numbers separated by a comma:\n').strip()
     unsorted = [int(item) for item in user_input.split(',')]
-    print(merge_sort(unsorted))
+    print(*merge_sort(unsorted), sep=',')

sorts/merge_sort_fastest.py

@@ -1,19 +1,46 @@
 '''
-Python implementation of merge sort algorithm.
+Python implementation of the fastest merge sort algorithm.
 Takes an average of 0.6 microseconds to sort a list of length 1000 items.
 Best Case Scenario : O(n)
 Worst Case Scenario : O(n)
 '''
-def merge_sort(LIST):
-    start = []
-    end = []
-    while len(LIST) > 1:
-        a = min(LIST)
-        b = max(LIST)
-        start.append(a)
-        end.append(b)
-        LIST.remove(a)
-        LIST.remove(b)
-    if LIST: start.append(LIST[0])
+from __future__ import print_function
+
+
+def merge_sort(collection):
+    """Pure implementation of the fastest merge sort algorithm in Python
+
+    :param collection: some mutable ordered collection with heterogeneous
+    comparable items inside
+    :return: a collection ordered by ascending
+
+    Examples:
+    >>> merge_sort([0, 5, 3, 2, 2])
+    [0, 2, 2, 3, 5]
+
+    >>> merge_sort([])
+    []
+
+    >>> merge_sort([-2, -5, -45])
+    [-45, -5, -2]
+    """
+    start, end = [], []
+    while len(collection) > 1:
+        min_one, max_one = min(collection), max(collection)
+        start.append(min_one)
+        end.append(max_one)
+        collection.remove(min_one)
+        collection.remove(max_one)
     end.reverse()
-    return (start + end)
+    return start + collection + end
+
+
+if __name__ == '__main__':
+    try:
+        raw_input          # Python 2
+    except NameError:
+        raw_input = input  # Python 3
+
+    user_input = raw_input('Enter numbers separated by a comma:\n').strip()
+    unsorted = [int(item) for item in user_input.split(',')]
+    print(*merge_sort(unsorted), sep=',')