How do you find a peak element in an array? | Interview Question

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

**Target Audience:** Beginners preparing for coding interviews — you should know what an array and a loop are; binary search is explained from scratch. **Difficulty:** Medium **Language:** Python 3 > **How to read this doc:** Each idea is explained in plain language first, then tagged with `📌 Interview term:` so you also learn the exact words an interviewer expects. Every output, step count and ordering claim below was produced by actually running the Python programs in the **Verified** section. ## 1. Why This Even Matters — A Story First You are hiking in thick fog. You cannot see the mountain range, only the ground one step to your left and one step to your right. Your task is not to find the *highest* mountain — just to stand on *a* summit, any spot where both sides go down. The slow plan is to walk the whole trail from one end and stop at the first spot where the ground drops. The clever plan: look at the ground just ahead. If it goes **up**, keep walking that way — a path tha

The problem

Question presented to candidate: "Given an integer array where no two neighbours are equal, return the index of any peak — an element bigger than both its neighbours, with the outside of the array counting as minus infinity. Please write it in Python and aim for better than linear time."

What a strong answer should cover:

A peak always exists because out-of-range neighbours are minus infinity; any peak is accepted, not only the maximum.
The O(n) scan first, then binary search on the slope: compare nums[mid] with nums[mid + 1] and keep the half that must contain a peak.
The loop invariant that justifies it, and why the loop is while lo < hi with hi = mid.
O(log n) time and O(1) extra space; why the global maximum cannot be found this fast.
Python specifics: // for integer division (/ returns a float that cannot index a list), no integer overflow in (lo + hi) // 2, and an explicit check for an empty list.

Clarifying questions expected:

"Any peak or the highest one?"
"Can neighbours be equal?"
"What should an empty list return?"

Code / implementation expected: Yes — a short iterative find_peak(nums) function in Python.

binary-search
arrays

Try it yourself

Sketch your approach before you read the solution. That is what the interview tests.

Stuck? Get a hint.

A short nudge toward the approach. It will not give the answer away.

Solution

Target Audience: Beginners preparing for coding interviews — you should know what an array and a loop are; binary search is explained from scratch. Difficulty: Medium Language: Python 3

How to read this doc: Each idea is explained in plain language first, then tagged with 📌 Interview term: so you also learn the exact words an interviewer expects. Every output, step count and ordering claim below was produced by actually running the Python programs in the Verified section.

1. Why This Even Matters — A Story First

You are hiking in thick fog. You cannot see the mountain range, only the ground one step to your left and one step to your right. Your task is not to find the highest mountain — just to stand on a summit, any spot where both sides go down.

The slow plan is to walk the whole trail from one end and stop at the first spot where the ground drops. T

The solution is hidden so you can try first

9 min read · 13 sections

Python code

Edit the code and run it here. Each printed line with an expected value becomes a test.

PythonFind a peak on six inputs, including edge cases

Practice

PythonTrace the search, brute-force check 1,456 arrays, and count the work

Practice

Discussion

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