freeCodeCamp Challenge Guide: Pairwise

Guide
1

Pairwise

Solutions

Solution 1 (Click to Hide/Show) 
function pairwise(arr, arg) {
  let pairIndices = [];
  // Check every pair
  for (let i = 0; i < arr.length; i++) {
    for (let j = i + 1; j < arr.length; j++) {
      // Exclude pairs that contain previously paired elements
      if (arr[i] + arr[j] == arg
          && !pairIndices.includes(i)
          && !pairIndices.includes(j)) {
        pairIndices.push(i, j);
        break;
      }
    }
  }

  return pairIndices.reduce((sum, curr, index) => sum + curr, 0);
}

pairwise([1,4,2,3,0,5], 7);
Solution 2 (Click to Hide/Show) 
function pairwise(arr, arg) {
  let pairIndices = {};
  let sum = 0;

  // Potential matches are stored in `pairIndices`
  //
  // For example, if
  //  arr = [1, 4, 2, 3, 2, 5]
  //  arg = 7
  //
  // then after the first two loop iterations
  //
  //  pairIndices = {6: [0], 3: [1]}
  //
  // because 1 = 7 - 6 is at index 0
  //  and    4 = 7 - 3 is at index 1
  //
  // The object has an array of indices to handle
  // duplicate entries in `arr`
  for (let i = 0; i < arr.length; i++) {
    if (pairIndices[arr[i]]) {
      // add indices to sum if matching pair found
      sum += i + pairIndices[arr[i]].shift();
      if (pairIndices[arr[i]].length === 0) {
        delete pairIndices[arr[i]];
      }
    } else {
      // mark index as posibility if no matching pair
      pairIndices[arg - arr[i]] = pairIndices[arg - arr[i]] || [];
      pairIndices[arg - arr[i]].push(i);
    }
  }

  return sum;
}
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