From 2020-11-29 cassidy@cassidoo.co newsletter challenge Given an array of integers and a target value, return the number of pairs of array elements that have a difference equal to a target value.

arraydiff.js

/*
From 2020-11-29 cassidy@cassidoo.co newsletter challenge

Given an array of integers and a target value, return the number of pairs of array 
elements that have a difference equal to a target value.


Examples:
$ arrayDiff([1, 2, 3, 4], 1)
$ 3 // 2 - 1 = 1, 3 - 2 = 1, and 4 - 3 = 1
*/

function arrayDiffBrute(arr, target) {
  let pairCount = 0;
  for (let i = 0; i < arr.length; i++) {
    for (let j = i + 1; j < arr.length; j++) {
      if (Math.abs(arr[i] - arr[j]) === target) {
        pairCount++;
      }
    }
  }

  return pairCount;
}

function arrayDiff(arr, target) {
  let pairCount = 0;
  let valToIndex = {};
  for (let i = 0; i < arr.length; i++) {
    if (!valToIndex[arr[i]]) {
      valToIndex[arr[i]] = []
    }
    valToIndex[arr[i]].push(i);
  }

  for (let i = 0; i < arr.length; i++) {
    diffTarget = arr[i] + target;
    if (valToIndex[diffTarget]) {
      pairCount += valToIndex[diffTarget].length;
    }
  }

  return pairCount;
}

const tests = [
  {
    arr: [1, 2, 3, 4],
    target: 1,
    expected: 3,
  },
  {
    arr: [1, 4],
    target: 1,
    expected: 0,
  },
  {
    arr: [1, 4],
    target: 3,
    expected: 1,
  },
  {
    arr: [1, 4, 9, 14],
    target: 5,
    expected: 2,
  },
];

for (const test of tests) {
  const result = arrayDiff(test.arr, test.target);
  console.log(`arr: ${test.arr}, expected: ${test.expected}, got: ${result}`);
}