Algorithms Problem Solving: Running Array Sum

This post is part of the Algorithms Problem Solving series.

Problem description

This is the Running Array Sum problem. The description looks like this:

Given an array nums. We define a running sum of an array as runningSum[i] = sum(nums[0]…nums[i]).

Return the running sum of nums.


Input: nums = [1,2,3,4]
Output: [1,3,6,10]

Input: nums = [1,1,1,1,1]
Output: [1,2,3,4,5]

Input: nums = [3,1,2,10,1]
Output: [3,4,6,16,17]


The idea here is to cache the sum of the numbers while iterating through the list. For each number, add to the sum and then add the sum to the result list.

def running_sum(nums):
    sum = 0
    result = []

    for num in nums:
        sum += num

    return result


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