Function partition

src/searching/quickselect.js:25–41

(arr, lo, hi, pivotIdx)

   */
  function quickselect(arr, n, lo, hi) {
    function partition(arr, lo, hi, pivotIdx) {
      function swap(arr, i, j) {
        var temp = arr[i];
        arr[i] = arr[j];
        arr[j] = temp;
      }
      var pivot = arr[pivotIdx];
      swap(arr, pivotIdx, hi);
      for (var i = lo; i < hi; i += 1) {
        if (arr[i] < pivot) {
          swap(arr, i, lo);
          lo += 1;
        }
      }
      swap(arr, hi, lo);
      return lo;
    }

    if (arr.length <= n) {
      return undefined;