The Peak Finder

An easy Python interview practice problem on DataDriven. Write the Python and run it against test cases, with instant feedback.

Domain
Python
Difficulty
easy
Seniority
L3
Asked in
Athenahealth

Problem

Given a list of integers, return the maximum value without using max(). Return None if empty.

Largest number in the list. Max() is not an option.

Example 1

Input:

nums = [3,7,2,9,1]

Output:

9

Example 2

Input:

nums = [-5,-1,-8]

Output:

-1

Worked solution and explanation

Why this problem exists in real interviews

This tests the most basic linear scan accumulation pattern. By prohibiting max(), interviewers check whether candidates understand how maximum-finding actually works under the hood.

Break down the requirements

Step 1: Initialize with the first element

Set the current maximum to nums[0]. This handles the case where all numbers are negative.

Step 2: Scan the rest of the list

Compare each element to the current maximum. Update if the element is larger.

Step 3: Return the maximum

After scanning all elements, the tracked value is the largest.

The solution

Linear scan with running maximum
def find_max(nums):
    if not nums:
        return None
    largest = nums[0]
    for i in range(1, len(nums)):
        if nums[i] > largest:
            largest = nums[i]
    return largest

Common follow-up questions