poj Matrix Chain Multiplication

Language: Default Matrix Chain Multiplication Time Limit: 1000MS   Memory Limit: 65536K Total Submissions: 2053   Accepted: 1297 Description Suppose you have to evaluate an expression like A*B*C*D*E where A,B,C,D and E are matrices.  Since matrix multiplication is associative, the order in which multiplications are performed is arbitrary. However, the number of elementary multiplications needed strongly depends on the evaluation order you choose.  For example, let A be a 50*10 matrix, B a 10*20 matrix and C a 20*5 matrix.  There are two different strategies to compute A*B*C, namely (A*B)*C and A*(B*C).  The first one takes 15000 elementary multiplications, but the second one only 3500.  Your job is to write a program that determines the number of elementary multiplications needed for a given evaluation strategy.  Input Input consists of two parts: a list of matrices and a list of expressions.  The first line of the input file contains one integer n (1 <= n <= 26), representing the number of matrices in the first part. The next n lines each contain one capital letter, specifying the name of the matrix, and two integers, specifying the number of rows and columns of the matrix.  The second part of the input file strictly adheres to the following syntax (given in EBNF):  SecondPart = Line { Line } Line = Expression Expression = Matrix | "(" Expression Expression ")" Matrix = "A" | "B" | "C" | ... | "X" | "Y" | "Z" Output For each expression found in the second part of the input file, print one line containing the word "error" if evaluation of the expression leads to an error due to non-matching matrices. Otherwise print one line containing the number of elementary multiplications needed to evaluate the expression in the way specified by the parentheses. Sample Input 9 A 50 10 B 10 20 C 20 5 D 30 35 E 35 15 F 15 5 G 5 10 H 10 20 I 20 25 A B C (AA) (AB) (AC) (A(BC)) ((AB)C) (((((DE)F)G)H)I) (D(E(F(G(HI))))) ((D(EF))((GH)I)) Sample Output 0 0 0 error 10000 error 3500 15000 40500 47500 15125 Source Ulm Local 1996 #include <cstdio> #include <iostream> #include <stack> #include <cstdio> using namespace std; struct Matrix { int a,b; }m[26]; int main() { int n; stack <Matrix> s; 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 st; while(cin>>st) { int len=st.length(); int error = 0; int ans=0; for(int i=0; i<len; i++) { if(isalpha(st[i])) s.push(m[st[i]-'A']); else if(st[i]==')') { Matrix m2=s.top(); s.pop(); Matrix m1=s.top(); s.pop(); if(m1.b==m2.a) { ans+=m1.a*m1.b*m2.b; m1.b=m2.b; s.push(m1); } else error=1; } } if(error) cout<<"error\n"; else cout<<ans<<endl; } return 0; }