diff --git a/note/001/README.md b/note/001/README.md
index f0224222..a93e6bc6 100644
--- a/note/001/README.md
+++ b/note/001/README.md
@@ -44,11 +44,12 @@ class Solution {
 ```java
 class Solution {
     public int[] twoSum(int[] nums, int target) {
-        int len = nums.length;
+        final int len = nums.length;
         HashMap<Integer, Integer> map = new HashMap<>();
         for (int i = 0; i < len; ++i) {
-            if (map.containsKey(nums[i])) {
-                return new int[]{map.get(nums[i]), i};
+            final Integer value = map.get(nums[i]);
+            if (value != null) {
+                return new int[] { value, i };
             }
             map.put(target - nums[i], i);
         }