The Running Total

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

Problem

Given a list of integers, return a list where position i is the sum of input[0..i] inclusive.

Each position holds the sum of everything before it.

Example

Input:

nums = [1,2,3,4,5]

Output:

[1,3,6,10,15]

Worked solution and explanation

Why this problem exists in real interviews

This tests the prefix sum pattern, the most fundamental cumulative operation in data processing. It checks whether candidates can build a running accumulator in a single pass.

Break down the requirements

Step 1: Initialize a running sum

Start at 0.

Step 2: Add each element and store the result

For each element, add it to the running sum and append the sum to the output.

The solution

Single-pass prefix sum accumulation
def running_total(nums):
    result = []
    total = 0
    for num in nums:
        total += num
        result.append(total)
    return result

Common follow-up questions