Good morning! Here's our prompt for today.

The setup is the same as Two Sum-- you're given an array of numbers, and a "goal" number.

Write a method to return an array of the indexes of the two elements in the array that sum up to the goal. If there are no such elements, return an empty array.

The caveat here is that the numbers are guaranteed to be sorted.

So let's say our goal number was 10. Our numbers to sum to it would be 3 and 7, and their indices 1 and 2 respectively.

1let arr = [1, 3, 7, 9, 11];
2let goal = 10;
3twoSum(arr, goal);
4// [1, 2]

Is there an efficient way to figure this out?

Constraints

• Length of the array <= 100000
• The array will always contain integer values between -1000000000 and 1000000000
• The required sum is will be well within the integer range
• Expected time complexity : O(n)
• Expected space complexity : O(1)

Try to solve this here or in Interactive Mode.

How do I practice this challenge?

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var assert = require('assert');

​

function sortedTwoSum(nums, goal) {

  return nums;

}

​

let arr = [1, 3, 7, 8, 11];

let goal = 10;

console.log(sortedTwoSum(arr, goal));

​

try {

  assert.deepEqual(sortedTwoSum([3, 6, 13, 14], 16), [0, 2]);

​

  console.log(

    'PASSED: <code>sortedTwoSum([3, 6, 13, 14], 16)</code> should return <code>[0, 2]</code>'

  );

} catch (err) {

  console.log(err);

}

​

try {

  assert.deepEqual(sortedTwoSum([1, 9, 13, 20, 47], 67), [3, 4]);

​

  console.log(

    'PASSED: <code>sortedTwoSum([1, 9, 13, 20, 47], 67)</code> should return <code>[3, 4]</code>'

  );

} catch (err) {

  console.log(err);

}

Here's a video of us explaining the solution.

To change the speed of the video or see it in full screen, click the icons to the right of the progress bar.

Here's our guided, illustrated walk-through.

How do I use this guide?