/*
  QUESTION:
    Given an array of integers, and a number ‘sum’,
    return a Boolean denoting whether there is a pair
    of integers in the array whose sum is equal to ‘sum’.
  NOTE:
    Expected time complexity O(n)
*/
const arr = [-1,1,2,5,3,4,5]
const hasPair = (arr, sum) => {
  const comps = new Set() // set of complimenting pairs
  return arr.some(item => comps.has(item) || (comps.add(sum-item), false))
}
hasPair(arr, -1)  // false
hasPair(arr, 0)   // true
hasPair(arr, 8)   // true
hasPair(arr, 10)  // true
hasPair(arr, 11)  // false