uva 442 Matrix Chain Multiplication

#include<iostream>#include<cstdio>#include<cstring>#include<stack>using namespace std;struct Matrix{    int a,b;    Matrix(int a=0,int b=0):a(a),b(b) {};}m[26];stack<Matrix> s;int main(){    int n;    cin>>n;    for(int i=0;i<n;i++)    {        string name;        cin>>name;        int k=name[0]-'A';        cin>>m[k].a>>m[k].b;    }    string expr;    while(cin>>expr)    {        int len=(int)expr.length();        bool error=false;        int ans=0;        for(int i=0;i<len;i++)        {            if(isalpha(expr[i])) s.push(m[expr[i]-'A']);            else if(expr[i]==')'){                Matrix m1=s.top();s.pop();                Matrix m2=s.top();s.pop();                if(m2.b!=m1.a) {error=true;break;}                ans+=m2.a*m2.b*m1.b;                s.push(Matrix(m2.a,m1.b));            }        }        if(error) cout<<"error"<<endl;        else cout<<ans<<endl;    }    return 0;}