# Leetcode Problem Review #1

I’ve decided that I will start reviewing some of the algorithm problems I come across while practicing on LeetCode.

# Problem 1480: Running Sum of 1d Array

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`

.

**Example**

**Input:** nums = [1,2,3,4]**Output:** [1,3,6,10]**Explanation:** Running sum is obtained as follows: [1, 1+2, 1+2+3, 1+2+3+4].

## Constraints

`1 <= nums.length <= 1000`

`-10^6 <= nums[i] <= 10^6`

var runningSum = function(nums){};

# My Process

## Understanding the Problem

The first thing I did before coding anything was to make sure I understood what the problem was. The examples provided did a pretty good job at explaining what they wanted from me. The input — **nums **— for this function is an array of random numbers between negative 1 million and positive 1 million and the length of the array is between 1 and 1000 elements. The output should be an array of the running total from each summation of the input elements.

## Breaking it Down

I then started making writing comments in my function about what I need returned and that I need to “do something.” We’ll break down what that “something” is soon.

var runningSum = function(nums){//DO SOMETHING...//RETURN AN ARRAY THAT HAS THE RUNNING SUM OF THE INPUT ARRAY};

My thought process was that I needed to have an empty array to begin with so I can push in the sums one by one and then return that array at the end of the function.

var runningSum = function(nums){

let totalArray = []//DO SOMETHING...//RETURN AN ARRAY THAT HAS THE RUNNING SUM OF THE INPUT ARRAYreturn totalArray};

Now about the “do something” comment. I felt like I should use a *for loop* through each element in the **nums** input array, that way I could do some kind of summation while working on one element at a time. I will expand in this soon.

var runningSum = function(nums){

let totalArray = []for (let i = 0; i < nums.length; i++ ){//DO SOMETHING IN LOOP...}//RETURN AN ARRAY THAT HAS THE RUNNING SUM OF THE INPUT ARRAYreturn totalArray};

So now that at this point I started to think about how I would push a running sum of numbers into **totalArray**. I can see how running sums work by the examples provided. If our input is [1,2,3,4] our output is [1,3,6,10] because…

**current sum | current element | new sum (pushed into array)**

----------------------------------------------------------

0 **|** 1 ** | ** 1

1 **|** 2 **|** 3

3 **|** 3 **|** 6

6 **|** 4 **|** 10

So in our function I’m going to create a variable called **currentSum** and set that to 0 as the starting point since we haven’t started adding anything yet. Then as the function goes through the for loop, I’m going to add the* current element* to the **currentSum** variable and push that *new sum* into the **totalArray** variable.

var runningSum = function(nums){

let totalArray = []//SET currentSum VARIABLE TO 0let currentSum = 0for (let i = 0; i < nums.length; i++ ){//ADD THE CURRENT ELEMENT TO currentSumcurrentSum += nums[i]//PUSH THE currentSum INTO TOTAL ARRAY AFTER THE CURRENT ELEMENT IS ADDED TO currentSumtotalArray.push(currentSum)}//RETURN AN ARRAY THAT HAS THE RUNNING SUM OF THE INPUT ARRAYreturn totalArray};

## SOLVED

var runningSum = function(nums){

let totalArray = []

let currentSum = 0for (let i = 0; i < nums.length; i++ ){currentSum += nums[i]totalArray.push(currentSum)

}return totalArray};

# My Thoughts…

It took me some time to complete this problem. I overthought a lot of things the first time I did this problem because I did not fully understand the problem. I overthought everything so much that I was about to put a loop inside of a loop with O notation of O(n²) which would be ridiculous for a simple problem like this. That’s when I stopped and started over from scratch.

I learned it’s best to make sure you 100% what is being asked of you and to pseudocode everything you can and work using a simple input that makes more sense to you. After you’re able to make that work, expand and try using a more complex input.

I’m hoping that sharing my detailed problem-solving process helps someone form their own process that works great with them.