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* add prefix sum * updating DIRECTORY.md * [pre-commit.ci] auto fixes from pre-commit.com hooks for more information, see https://pre-commit.ci Co-authored-by: github-actions <${GITHUB_ACTOR}@users.noreply.github.com> Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>
79 lines
1.9 KiB
Python
79 lines
1.9 KiB
Python
"""
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Author : Alexander Pantyukhin
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Date : November 3, 2022
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Implement the class of prefix sum with useful functions based on it.
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"""
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class PrefixSum:
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def __init__(self, array: list[int]) -> None:
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len_array = len(array)
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self.prefix_sum = [0] * len_array
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if len_array > 0:
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self.prefix_sum[0] = array[0]
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for i in range(1, len_array):
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self.prefix_sum[i] = self.prefix_sum[i - 1] + array[i]
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def get_sum(self, start: int, end: int) -> int:
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"""
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The function returns the sum of array from the start to the end indexes.
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Runtime : O(1)
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Space: O(1)
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>>> PrefixSum([1,2,3]).get_sum(0, 2)
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6
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>>> PrefixSum([1,2,3]).get_sum(1, 2)
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5
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>>> PrefixSum([1,2,3]).get_sum(2, 2)
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3
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>>> PrefixSum([1,2,3]).get_sum(2, 3)
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Traceback (most recent call last):
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...
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IndexError: list index out of range
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"""
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if start == 0:
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return self.prefix_sum[end]
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return self.prefix_sum[end] - self.prefix_sum[start - 1]
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def contains_sum(self, target_sum: int) -> bool:
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"""
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The function returns True if array contains the target_sum,
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False otherwise.
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Runtime : O(n)
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Space: O(n)
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>>> PrefixSum([1,2,3]).contains_sum(6)
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True
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>>> PrefixSum([1,2,3]).contains_sum(5)
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True
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>>> PrefixSum([1,2,3]).contains_sum(3)
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True
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>>> PrefixSum([1,2,3]).contains_sum(4)
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False
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>>> PrefixSum([1,2,3]).contains_sum(7)
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False
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>>> PrefixSum([1,-2,3]).contains_sum(2)
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True
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"""
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sums = {0}
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for sum_item in self.prefix_sum:
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if sum_item - target_sum in sums:
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return True
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sums.add(sum_item)
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return False
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if __name__ == "__main__":
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import doctest
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doctest.testmod()
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