cassidoo 11/29/2020

arrayDiff.js

function arrayDiff(arr, target) {
  const seen = {};
  return arr.reduce((numPairs, n) => {
    const targetsFromN = { [n + target]: null, [n - target]: null };
    numPairs += Object.keys(targetsFromN).reduce(
      (seenFromN, targetFromN) => seenFromN + (seen[targetFromN] || 0),
      0
    );
    seen[n] = (seen[n] || 0) + 1;
    return numPairs;
  }, 0);
}