diff --git a/problems/0020.有效的括号.md b/problems/0020.有效的括号.md
index dbbd258a..7972995d 100644
--- a/problems/0020.有效的括号.md
+++ b/problems/0020.有效的括号.md
@@ -17,7 +17,7 @@ public:
             else if (s[i] == '{') st.push('}');
             else if (s[i] == '[') st.push(']');
             else if (st.empty() || st.top() != s[i]) return false;
-            else st.pop();
+            else st.pop(); // st.top() == s[i]
         }
         return st.empty();
     }