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fix clrs-07-01-02 solution bug #36

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fix clrs-07-01-02 solution bug #36

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3 changes: 1 addition & 2 deletions other/clrs/07/01/02.markdown
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Original file line number Diff line number Diff line change
Expand Up @@ -5,5 +5,4 @@

It returns $r$.

We can modify `PARTITION` by counting the number of comparisons in which $A[j]
= A[r]$ and then subtracting half that number from the pivot index.
We can modify `PARTITION` by moving numbers smaller than the pivot to the leftmost and numbers equal to the pivot after the smaller group. At last we return the count of smaller group plus half of count the equal numbers.
18 changes: 10 additions & 8 deletions other/clrs/07/01/02.py
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Original file line number Diff line number Diff line change
Expand Up @@ -8,15 +8,17 @@ def partition(numbers, start = 0, end = None):
for i in range(start, last):
value = numbers[i]

if value == pivot_value:
repetitions += 1

if value <= pivot_value:
numbers[pivot], numbers[i] = numbers[i], numbers[pivot]
if value < pivot_value:
numbers[pivot+repetitions], numbers[i] = numbers[i], numbers[pivot+repetitions]
numbers[pivot+repetitions], numbers[pivot] = numbers[pivot], numbers[pivot+repetitions]
pivot += 1

numbers[pivot], numbers[last] = numbers[last], numbers[pivot]
return pivot - repetitions // 2
elif value == pivot_value:
numbers[pivot+repetitions], numbers[i] = numbers[i], numbers[pivot+repetitions]
numbers[pivot+repetitions], numbers[pivot] = numbers[pivot], numbers[pivot+repetitions]
repetitions += 1

numbers[pivot+repetitions], numbers[last] = numbers[last], numbers[pivot+repetitions]
return pivot + repetitions // 2

def quicksort(numbers, start = 0, end = None):
end = end if end else len(numbers)
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5 changes: 3 additions & 2 deletions other/clrs/07/01/02.test.py
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Expand Up @@ -15,8 +15,9 @@ def test_normal_partition(self):
self.assertEqual(numbers, [3, 5, 8, 7, 4, 2, 6, 11, 21, 13, 19, 12])

def test_partition_with_repetition(self):
self.assertEqual(partition([2, 2, 2, 2, 2, 2]), 3)
self.assertEqual(partition([1, 2, 2, 2, 3, 2]), 3)
self.assertEqual(partition([2, 2, 2, 2, 2, 2]), 2)
self.assertEqual(partition([1, 2, 2, 2, 3, 2]), 2)
self.assertEqual(partition([5, 2, 4, 7, 5, 1, 5, 9, 5, 1, 10, 5]), 6)

def test_quicksort(self):
numbers = [13, 19, 3, 5, 12, 8, 7, 4, 21, 2, 6, 11]
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