diff --git a/notes/算法 - 排序.md b/notes/算法 - 排序.md
index da0d046e..ed678c5a 100644
--- a/notes/算法 - 排序.md	
+++ b/notes/算法 - 排序.md	
@@ -363,6 +363,7 @@ public class ThreeWayQuickSort<T extends Comparable<T>> extends QuickSort<T> {
 
 ```java
 public T select(T[] nums, int k) {
+    shuffle(nums);
     int l = 0, h = nums.length - 1;
     while (h > l) {
         int j = partition(nums, l, h);
@@ -397,6 +398,7 @@ public class Heap<T extends Comparable<T>> {
     private T[] heap;
     private int N = 0;
 
+    // 容量固定为 maxN，插入超过 maxN 个元素时数组越界
     public Heap(int maxN) {
         this.heap = (T[]) new Comparable[maxN + 1];
     }
@@ -471,6 +473,8 @@ public void insert(T v) {
 
 ```java
 public T delMax() {
+    if (isEmpty())
+        throw new NoSuchElementException("堆为空");
     T max = heap[1];
     swap(1, N--);
     heap[N + 1] = null;