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+# merge sort
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+# http://interactivepython.org/courselib/static/pythonds/SortSearch/TheMergeSort.html
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+
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+import pytest
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+
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+def test_merge_sort():
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+ input1 = [31, 23, 43, 56, 9]
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+ input2 = []
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+ assert merge_sort(input1) == [9, 23, 31, 43, 56]
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+ assert merge_sort(input2) == []
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+
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+def merge_sort(input):
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+ if len(input) > 1:
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+ mid = len(input)//2
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+ left = input[:mid]
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+ right = input[mid:]
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+
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+ merge_sort(left)
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+ merge_sort(right)
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+
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+ # in-place merge, the merge always starts from the left as
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+ # merge_sort(left) is called first
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+ i,j,k = 0,0,0
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+ # sync compare
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+ while i < len(left) and j < len(right):
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+ if left[i] < right[j]:
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+ input[k] = left[i]
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+ i += 1
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+ else:
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+ input[k] = right[j]
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+ j += 1
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+ k += 1
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+ # the larger ones
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+ while i < len(left):
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+ input[k] = left[i]
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+ i += 1
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+ k += 1
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+ while j < len(right):
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+ input[k] = right[j]
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+ j += 1
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+ k += 1
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+ return input
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