function pairs(array, target) {
  // Cache to hold visited elements as keys and index as value O(n) space
	const cache = {};

  // O(n) iteration of array
	return array.reduce((output, val, i) => {
	    let diff = target - val;

	    if (cache.hasOwnProperty(diff)) {
	      output.push([cache[diff], i]);
	    }

	    cache[val] = i;

	    return output;
	}, []);
}