diff --git a/src/chapter1/ch1-q1.js b/src/chapter1/ch1-q1.js
index 63a2dcc..8e91c47 100644
--- a/src/chapter1/ch1-q1.js
+++ b/src/chapter1/ch1-q1.js
@@ -35,10 +35,10 @@ export function hasUniqueCharactersSet(str) {
  */
 export function hasUniqueCharactersSort(str) {
   // sort string using quicksort
-  str.sort();
+  var sortStrArr = str.split('').sort();
 
-  for (var i = 1; i < str.length; ++i) {
-    if (str[i] === str[i - 1]) {
+  for (var i = 1; i < sortStrArr.length; ++i) {
+    if (sortStrArr[i] === sortStrArr[i - 1]) {
       return false;
     }
   }