sum-of-pairs.js

// Sum of Pairs
// Given a list of integers and a single sum value, return the first two values (parse from the left please) in order of appearance that add up to form the sum.
// 
// sum_pairs([11, 3, 7, 5],         10)
// #              ^--^      3 + 7 = 10
// == [3, 7]
// 
// sum_pairs([4, 3, 2, 3, 4],         6)
// #          ^-----^         4 + 2 = 6, indices: 0, 2 *
// #             ^-----^      3 + 3 = 6, indices: 1, 3
// #                ^-----^   2 + 4 = 6, indices: 2, 4
// #  * entire pair is earlier, and therefore is the correct answer
// == [4, 2]
// 
// sum_pairs([0, 0, -2, 3], 2)
// #  there are no pairs of values that can be added to produce 2.
// == None/nil/undefined (Based on the language)
// 
// sum_pairs([10, 5, 2, 3, 7, 5],         10)
// #              ^-----------^   5 + 5 = 10, indices: 1, 5
// #                    ^--^      3 + 7 = 10, indices: 3, 4 *
// #  * entire pair is earlier, and therefore is the correct answer
// == [3, 7]
// Negative numbers and duplicate numbers can and will appear.
// 
// NOTE: There will also be lists tested of lengths upwards of 10,000,000 elements. Be sure your code doesn't time out.

function sumPairs(ints, s){
  let secondPossiblePairs = new Set();
  let n;
  for (let i = 0; i<ints.length; i++){
    n = ints[i];
    if (secondPossiblePairs.has(s - n)){
      return [s - n, n];
    }
    secondPossiblePairs.add(n);
  }
    
  return undefined;
}