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49 lines
1.1 KiB
Python
49 lines
1.1 KiB
Python
"""
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Author : Siddharth Warrier
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Date : October 3, 2023
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Task:
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Count the no of pairs in a given array with given sum
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Implementation notes: Using hashing
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The idea is that we hash the array in a dictionary
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Then go through the elements of the array
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We subtract this with the given sum
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and check if that is there in the array
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We also check the edge cases like if there are multiple same elements
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Finally we divide the count by 2
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to avoid the same pair getting counted twice
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"""
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def pairs_with_sum(arr: list, req_sum: int) -> int:
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"""
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Return the no. of pairs with sum "sum"
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>>> pairs_with_sum([1,5,7,1],6)
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2
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>>> pairs_with_sum([1,1,1,1,1,1,1,1],2)
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28
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>>> pairs_with_sum([1,7,6,2,5,4,3,1,9,8],7)
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4
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"""
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d: dict = {}
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for i in arr:
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if i in d:
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d[i] += 1
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else:
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d[i] = 1
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ans = 0
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for i in arr:
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d[i] -= 1
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if req_sum - i in d and d[req_sum - i] != 0:
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ans += d[req_sum - i]
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d[i] += 1
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return ans // 2
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if __name__ == "__main__":
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import doctest
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doctest.testmod()
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