Good afternoon! Here's our prompt for today.

This is a classic and very common interview problem. Given an array of integers, return the indices of the two numbers in it that add up to a specific "goal" number.

Suppose we had an array [1, 3, 6, 7, 9], and let's say our "goal" number was 10. Our numbers to sum to it could be 3 and 7, and we would return an array of indices 1 and 3 respectively.

1let arr = [1, 3, 6, 7, 9];
2let goal = 10;
3twoSum(arr, goal);
4// [1, 3]

You may assume that each input would have exactly one solution. Additionally, you may not use the same element twice towards the sum. This means if given [1, 3] and a goal of 2, you cannot use 1 twice and return its index twice as [0, 0].

Note that you cannot guarantee the array is sorted.

AlgoDaily partner CodeTips.co.uk has kindly provided a guide on solving Two Sum in Go. Check it out for even deeper coverage of this problem.

Constraints

• Length of the array <= 100000
• The values of the array will be between -1000000000 and 1000000000
• The array can be empty
• The target sum will be between -1000000000 and 1000000000
• Expected time complexity : O(n)
• Expected space complexity : O(n)

Try to solve this here or in Interactive Mode.

How do I practice this challenge?

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var assert = require('assert');

​

function twoSum(arr, goal) {

  const results = [];

  // define this method

  return results;

}

​

try {

  assert.deepEqual(twoSum([1, 9, 13, 20, 47], 10), [0, 1]);

​

  console.log(

    'PASSED: ' + 'assert.deepEqual(twoSum([1, 9, 13, 20, 47], 10), [0, 1]);'

  );

} catch (err) {

  console.log(err);

}

​

try {

  assert.deepEqual(twoSum([3, 2, 4, 1, 9], 12), [0, 4]);

​

  console.log(

    'PASSED: ' + 'assert.deepEqual(twoSum([3, 2, 4, 1, 9], 12), [0, 4]);'

  );

} catch (err) {

  console.log(err);

}

​

try {

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Here's how we would solve this problem...

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