Update Leetcode 题解.md

docs/notes/Leetcode 题解.md

@@ -403,12 +403,14 @@ public List<Integer> topKFrequent(int[] nums, int k) {
     }
     List<Integer> topK = new ArrayList<>();
     for (int i = buckets.length - 1; i >= 0 && topK.size() < k; i--) {
+        if (buckets[i] != null) {
             if (buckets[i].size() <= (k - topK.size())) {
                 topK.addAll(buckets[i]);
             } else {
                 topK.addAll(buckets[i].subList(0, k - topK.size()));
             }
         }
+    }
     return topK;
 }
 ```