Pairwise: Given an array arr, find element pairs whose sum equal the second argument arg and return the sum of their indices. You may use multiple pairs that have the same numeric elements but different indices. Each pair should use the lowest possible available indices. Once an element has been used it cannot be reused to pair with another elem…

pairwise.js

function pairwise(arr, arg) {
  let sum = 0;
  for(let i=0; i<arr.length-1; i++){
    for(let j=i+1; j<arr.length; j++){
      if(arr[i] + arr[j] === arg){
        sum += (i+j);
        arr[i] = arg + 1; // to avoid the arr[i] being reused
        arr[j] = arg + 1; // to avoid the arr[j] being reused
      }
    }
  }
  return sum;
}

pairwise([1,4,2,3,0,5], 7); // return 11

/*
pairwise([1, 3, 2, 4], 4) should return 1.
Passed
pairwise([1, 1, 1], 2) should return 1.
Passed
pairwise([0, 0, 0, 0, 1, 1], 1) should return 10.
Passed
pairwise([], 100) should return 0.
 */