Facebook Interview: Flatten an array

solution.js

/* 
  Inputs
  -------
    (1): Empty array
    --
    []
    => []

    (2): Flattened already
    --
    [1]
    => [1]

    (3): One level deep
    --
    [[1]]
    => [1]

    (4): Mixed with one level deep
    --
    [1, [2]]
    => [1, 2]

    (5): One level deep multiple arrays
    --
    [[1, 2], [3, 4]]
    => [1, 2, 3, 4]

    (6): Two levels deep
    --
    [[1, [2, 3]], [4, [5, 6, 7]]]
    => [1, 2, 3, 4, 5, 6, 7]

    (7): Arbitrary depth
    --
    [1, [2, [[3]], [4, 5, 6]], [[[[[[7, 8, 9]]]]]]]
    => [1, 2, 3, 4, 5, 6, 7, 8, 9]
*/

function flatten(arr) {
  return arr.reduce((flatArray, current) => {
    if (Array.isArray(current)) {
      return flatArray.concat(flatten(current));
    }
    
    flatArray.push(current);
    return flatArray;
  }, []);
}

function flattenIterative(arr) {
  const flatArray = [];
  let original = [...arr];
  
  while (original.length > 0) {
    const current = original.shift();
    
    if (Array.isArray(current)) {
      original = current.concat(original);
    } else {
      flatArray.push(current);
    }
  }
  
  return flatArray;
}

const input1 = [];
console.log('result', flatten(input1));
console.log('result', flattenIterative(input1));

const input2 = [1];
console.log('result', flatten(input2));
console.log('result', flattenIterative(input2));

const input3 = [[1]];
console.log('result', flatten(input3));
console.log('result', flattenIterative(input3));

const input4 = [1, [2]];
console.log('result', flatten(input4));
console.log('result', flattenIterative(input4));

const input5 = [[1, 2], [3, 4]];
console.log('result', flatten(input5));
console.log('result', flattenIterative(input5));

const input6 = [[1, [2, 3]], [4, [5, 6, 7]]];
console.log('result', flatten(input6));
console.log('result', flattenIterative(input6));

const input7 = [1, [2, [[3]], [4, 5, 6]], [[[[[[7, 8, 9]]]]]]];
console.log('result', flatten(input7));
console.log('result', flattenIterative(input7));