sort

//Quick sort
//Time Complexity

function quickSort(arr) {
  if(arr.length < 2) {return arr;}
  var middleIndex = Math.floor(arr.length/2);
  var left = [];
  var right = [];
  for(var k=0; k<arr.length; k++){
    if(arr[k] <= arr[middleIndex]) { left.push(arr[k]);}
    else if(arr[middleIndex] <arr[k]) { right.push(arr[k]);}
  }
  
  var _left = quickSort(left);
  var _right = quickSort(right);
  
  return _left.concat(_right);
}

// from online:

function quickSort(a) {
  if (a.length <= 1) return a;

  var mid = ~~(a.length / 2)
    , midItem = a.splice(mid, 1)[0]
    , left = []
    , right = [];

  a.forEach(function(item) {
    if (item <= midItem)
      left.push(item);
    else 
      right.push(item);
  });

  var _left = quickSort(left)
    , _right = quickSort(right);

  return _left.concat(midItem, _right);
}