Update 6.quickSort.md

Author: Rain120

6.quickSort.md

@@ -97,6 +97,10 @@ function partition2(arr, low, high) {
 }
 
 function quickSort2(arr, low, high) {
+  const len = arr.length;
+  low = typeof low != 'number' ? 0 : low,
+  high = typeof high != 'number' ? len - 1 : high;
+
   if (low < high) {
     let pivot = partition2(arr, low, high);
     quickSort2(arr, low, pivot - 1);