The Coding Interview: Find the Symmetric Difference

This series answers coding interview questions from the Coding Interview Prep collection at freeCodeCamp. You can find more with the interview questions tag.

The concept is to find the numbers in two arrays that do not repeat in both. So [1,2] and [2,3] would produce [1,3]. If you’re comparing more than 2 arrays, you compare the next array (and beyond) to the result of the last comparison. So [1,2], [2, 3], and [2,4] would produce [1,2,3,4] because 2 is filtered out of the first comparison and it isn’t in the result being compared in the next, so it comes back.

Solution explained

First, the args argument (provided in the scaffolded function) only gives you the first array submitted as an argument. It should just be ignored. Use the arguments object instead to get an array of all the arguments.

• Create an array(result) to hold the results.

• Run through the arguments array with a loop (i). Start by filtering duplicates from the array using the spread operator and Set function.

• With another loop (j), check each number in the filtered set to see if it appears in result. If it’s not there, add it with push. If it’s already there, remove it with slice.

• The result of the first pass of the loop will add the first array to the loop in its entirety. The result of each successive pass will perform a symmetric compare between the next array (arguments[i]) with the result array.

• Finally, do a sort on result (requiring the function we pass to result.sort() as an argument to ensure a numeric sort instead of alphabetic) because the tests require the result to be sorted to pass.

Solution

function sym(args) {
  let result = [];
  for(let i in arguments){
    let filtered = [... new Set(arguments[i])];
    for(let j in filtered){
      let number = filtered[j];  
      if(result.indexOf(number) === -1){
          result.push(number);
        } else {
          result.splice(result.indexOf(number), 1);
        }
      }
    }
  result.sort(function(a, b) {
    return a - b;
  });
  return result;
}