1 bool isBalanced(string s) {
2  stack<char> st;
3  for (char c : s) {
4   if (c == '(' || c == '[' || c == '{') {
5    st.push(c);
6   } else {
7    if (st.empty()) return false; // 无左括号匹配
8    char top = st.top();
9    st.pop();
10    if ((c == ')' && top != '(') ||
11     (c == ']' && top != '[') ||
12     (c == '}' && top != '{')) {
13     return false;
14    }
15   }
16  }
17  return ________________; //在此处填入代码
18 }