Level 2: Arrays and objects · Exercise 29 of 76 · Free

Two numbers that add up

A JavaScript exercise from Scriptdrill. Solve it with a code editor in your browser that runs your function against example and hidden tests, and it is checked the moment you submit.

The task

Find the two different positions in `nums` whose values add up to `target` and return them as `[i, j]` with `i < j`. Every test has at most one such pair; return `null` when there is none. The same position cannot be used twice, but two equal values at different positions can. Do not change `nums`. One hidden test has 100,000 numbers, so comparing every pair is too slow.

What this exercise teaches

The Map turns the search for the partner into a single lookup, so the whole thing is one pass instead of a loop inside a loop. Checking the map before storing the current number is what stops a number from pairing with itself.

A hint if you get stuck

Walk the array once and keep a `Map` from each value you have seen to its index. For the current number, check whether `target - n` is already in the map before you add the number itself.