QuickSort

/*********DISCLAIMER***********/
/* there is more than one way to do a sort :) */


function swap(arr, idx1, idx2) {
  var temp = arr[idx1];
  arr[idx1] = arr[idx2];
  arr[idx2] = temp;
}

/* 1. Set the pivot value to be the value at the left index, and set a 
    varaible called partitionIndex equal to left. The partitionIndex will 
    help us keep track of where to perform our swaps so that we wind up with 
    values correctly placed on either side of the pivot.
  2. For every index greater than left and less than right + 1, compare the 
    array value to the pivot value.
  3. If the array value at the given index is less than the pivot value, 
    increment the partition index and swap the array value with the value at 
    the partition index.
  4. At the end, swap the pivot value with the value at the partition index 
    (this ensures that the pivot ends up in between values less than it and 
    values greater than it).
  5. Return the partition index. */
function partition(arr, left, right) {

  // 1
  var pivot = arr[left];
  var partitionIndex = left;

  // 2
  for(var i=left+1;i<right+1;i++){

    // 3
    if(arr[i]<pivot){
      partitionIndex++;
      swap(arr, i, partitionIndex);
    }
  }

  // 4
  swap(arr, left, partitionIndex);

  return partitionIndex;
}


function quickSort(arr, left=0, right=arr.length - 1) {
  // init ^^^
  /*
  1. If left is less than right, declare a variable called
  partitionIndex which is equal to the result of a call to
  partition, passing in arr, left, and right.

  After the call to partition, perform a quicksort to the
  two subarrays to the left and right of the partitionIndex.
  2. Return arr.
  */
  if(left < right){
    var partitionIndex = partition(arr,left,right);
    // what I thought
    quickSort(arr,left,partitionIndex-1);
    quickSort(arr,partitionIndex+1,right);
  }

  return arr;
}