Finds pairs of number in array which produce given sum.

find-pairs.js

// Finds pairs of number in array which produce given sum.

const ARRAY_SIZE = 1000;
const MAX_NUMBER = 2022;

const SUM_TO_FIND = 2022;

const arr = new Array(ARRAY_SIZE)
  .fill(0)
  .map(() => Math.floor(Math.random() * MAX_NUMBER));

// Sort array, so later we can sum small nums with large nums
const sortedArr = arr.sort((a, b) => a - b);

// Squash equal numbers into objects with count
const squashedArr = [];
for (let i = 0; i < sortedArr.length; i++) {
  if (i > 0 && squashedArr[squashedArr.length - 1].value === sortedArr[i]) {
    squashedArr[squashedArr.length - 1].count++;
  } else {
    squashedArr.push({ value: sortedArr[i], count: 1 });
  }
}

// Now we can do it in one run:
//
// If last num + first num < SUM_TO_FIND, we drop the first num
// If last num + first num > SUM_TO_FIND, we drop the last num
// If we found our sum, we drop the count on both nums and remove both nums
//
while (squashedArr.length > 1) {
  const last = squashedArr.length - 1;
  const sum = squashedArr[0].value + squashedArr[last].value;

  if (sum > SUM_TO_FIND) {
    squashedArr.pop();
    continue;
  }

  if (sum < SUM_TO_FIND) {
    squashedArr.shift();
    continue;
  }

  const dec = Math.min(squashedArr[0].count, squashedArr[last].count);
  squashedArr[0].count -= dec;
  squashedArr[last].count -= dec;

  console.log(
    `Found ${dec} pair${dec > 1 ? "s" : ""}:`,
    squashedArr[0].value,
    squashedArr[last].value
  );

  squashedArr.shift();
  squashedArr.pop();
}