Two-Sum - Notes | Ankitz007

Two-sum classic: store seen values in a map to get O(n) time.

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def two_sum(nums, target):
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  seen = {}
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  for i, v in enumerate(nums):
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    need = target - v
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    if need in seen:
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      return (seen[need], i)
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    seen[v] = i
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  return None
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# two_sum([2,7,11,15], 9) -> (0,1)

Edge cases: duplicate values, negative numbers — map handles them fine.