例题6-3 Matrix Chain Multiplication ，Uva 442

#include <cstdio>
#include <stack>
#include <cstring>
#include <cctype>
const int MAXN=1000+10;
using namespace std;
char exps[MAXN];
struct Matrix {
    int a, b;
    Matrix(int a = 0,int b = 0):a(a), b(b) {}
}m[26];
stack<Matrix> s;
int main(){
    #ifdef DEBUG
    freopen("6.3.in","r",stdin);
    #endif
    int n;
    scanf("%d\r",&n);
    for(int i=0;i<n;i++){
        char s0[10];
        char c;
        scanf("%c ",&c);
        s0[0]=c;
        scanf("%d %d\r\n",&m[s0[0] -‘A‘].a, &m[s0[0] -‘A‘].b);
        //printf("%c %d %d\r\n",s0[0] , m[s0[0] -‘A‘].a , m[s0[0]-‘A‘].b);
    }
    while(scanf("%s",exps)==1){
        int sum=0;
        int len=strlen(exps);
        int ok=1;
        for(int i=0;i<len;i++){
            if(isalpha(exps[i])){
                s.push(m[exps[i]-‘A‘]);
             //  printf("push %d %d \n",m[exps[i]-‘A‘].a,m[exps[i]-‘A‘].b);
            }
            else if(exps[i]==‘)‘){
                Matrix m2 = s.top(); s.pop();
            //    printf("pop %d %d \n", m2.a, m2.b);
                Matrix m1 = s.top(); s.pop();
             //   printf("pop %d %d \n", m1.a, m1.b);
                if(m1.b != m2.a){ok=0;break;}
                sum+= m1.a * m1.b * m2.b;
                s.push(Matrix(m1.a, m2.b));
             //  printf("push %d %d \n",m1.a, m2.b,);
            }
        }
        if(ok)printf("%d\n",sum);
        else printf("error\n");
    }
    return 0;
}