JavaScript: Two Sum

Two Sum is another question frequently asked during interviews. There are two common ways to approach this problem. One way is considered "brute force" - it uses nested loops with a time complexity of O(n²). The second way is one of the preferred methods, because it has a time complexity of O(n).

The problem asks: given an array of numbers and a target sum, find the sum of two numbers in the array that is equal to the target sum. This is how to solve it:

Step 1. Start with an array of numbers and a target sum

const array = [1, 2, 3, 4, 5, 6, 7, 8, 9];
const sum = 15; 

const twoSum = (array, sum) => {

};

Step 2. Write a for-loop to iterate through the array.

const twoSum = (array, sum) => {
    const past = []

    for(let i = 0; i < array.length; i++){
        let curr = array[i]
        let needed = sum - array[i]
    }
};

Step 3. Write if-statement to search through array of past numbers


const twoSum = (array, sum) => {
    const past = []

    for(let i = 0; i < array.length; i++){
        let curr = array[i]
        let needed = sum - array[i]
        if(!past.includes(needed)){
            past.push(curr)
        } else {
            return [needed, curr];
        }
    }
    return "Not found!"; 
};

Step 4. Add counter that will increment by one each run
This will show how many runs it takes to return "Not Found!"

const array = [1, 2, 3, 4, 5, 6, 7, 8, 9];
const sum = 20;

const twoSum = (array, sum) => {
    const past = [];
    let count = 0;

    for (let i = 0; i < array.length; i++) {
        let curr = array[i]
        let needed = sum - array[i]
        if (!past.includes(needed)) {
            past.push(curr)
        } else {
            return [needed, curr];
        }
    count++; 
    }
    console.log(count)
    return "Not found!";
};

and that's it!