Diff Two Arrays

Altcademy Team wrote on 7 February 2018

Compare two arrays and return a new array with any items only found in one of the two given arrays, but not both. In other words, return the symmetric difference of the two arrays.

比较两个数组，然后返回一个新数组，该数组的元素为两个给定数组中所有独有的数组元素。换言之，返回两个数组的差异。

function diff(arr1, arr2) {
  var newArr = [];
  // Same, same; but different.
  return newArr;
}

diff([1, 2, 3, 5], [1, 2, 3, 4, 5]);

The solution is O(n) by using hashes to keep tracking of item counts. If the item only has 1 count, that means it's unique between the two arrays.

function diff(arr1, arr2) {
  var newArr = [];
  var itemsCounter = {};
  
  // get arr1's items into itemsCounter
  arr1.forEach(function(item) {
    if (itemsCounter[item] === undefined) {
      itemsCounter[item] = 1;
    } else {
      itemsCounter[item]++;
    }
  });
  
  // get arr2's items into itemsCounter
  arr2.forEach(function(item) {
    if (itemsCounter[item] === undefined) {
      itemsCounter[item] = 1;
    } else {
      itemsCounter[item]++;
    }
  });
  
  Object.keys(itemsCounter).forEach(function(key){
    // if the item has only been counted once, that means that item is unique between arr1 and arr2
    if (itemsCounter[key] === 1) {
      newArr.push(key)
    }
  });
  return newArr;
}

diff([1, 2, 3, 5], [1, 2, 3, 4, 5]);