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All topics›JavaScript›Binary search

First and Last Position of a Value

intermediatelayer B6 · Binary search5 tests

Given a sorted array nums that may contain repeats, return a two-element array [first, last] holding the lowest and highest index where target occurs. Return [-1, -1] if it never occurs.

  • Two binary searches — one biased left, one biased right — beat scanning outward from a hit
  • A block of equal values can span the whole array
  • Must run in O(log n), so walking from a found index is not acceptable

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