括号匹配问题（百练ID3704）

((ABCD(x)
)(rttyy())sss)(

((ABCD(x)
$$
)(rttyy())sss)(
?         ?$
#include<iostream>
#include<stack>
#include<string>
#include<cstring>
using namespace std;
char a[101];
int main()
{stack<int>s;string str;while (cin>>str){memset(a, ' ', 100);for (int i = 0; i < str.size();++i){	if (str[i] == '(')s.push(i);if (str[i] == ')'){if (s.empty()){a[i] = '?';}else{s.pop();}}}while (!s.empty()){a[s.top()] = '$';s.pop();}cout << str << endl;cout << a <<endl;}return 0;
}