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How does binary search work and what are its requirements?

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01

Understand the problem

Explain binary search.

binary-search
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Binary search finds a target in a sorted array in O(log n) by repeatedly comparing the middle element and discarding half the range. Requirement: random-access, sorted data. Off-by-one errors in the lo/hi/mid bounds are the classic pitfall.

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04

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Iterative binary search
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def binary_search(arr, target):
    lo, hi = 0, len(arr) - 1
    while lo <= hi:
        mid = lo + (hi - lo) // 2
        if arr[mid] == target:
            return mid
        if arr[mid] < target:
            lo = mid + 1
        else:
            hi = mid - 1
    return -1


# --- demo ---
print(binary_search([-1, 0, 3, 5, 9, 12], 9))   # 4
print(binary_search([-1, 0, 3, 5, 9, 12], 2))   # -1
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