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How do you find a peak element in an array?

Q: How do you find a peak element in an array?

**Intuition.** A peak is any element greater than its neighbours. You don't have to scan — use **binary search on the slope**: if nums[mid] < nums[mid+1] , the array is rising, so a peak must lie to the **right**; otherwise it's to the left (including mid). You always walk uphill toward a peak. rising slope at mid → a peak is to the right <circle cx="

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Explain peak finding in O(log n).

binary-searcharrays

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Binary search toward the peak

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def find_peak(nums):
    lo, hi = 0, len(nums) - 1
    while lo < hi:
        mid = (lo + hi) // 2
        if nums[mid] < nums[mid + 1]:
            lo = mid + 1          # uphill to the right
        else:
            hi = mid              # peak is mid or to the left
    return lo


# --- demo ---
print(find_peak([1, 2, 3, 1]))   # 2  (nums[2] = 3 is a peak)

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