https://vjudge.net/PRoblem/UVA-442

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 Specification

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  (    ), 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 } <EOF>Line       = Expression <CR>Expression = Matrix | "(" Expression Expression ")"Matrix     = "A" | "B" | "C" | ... | "X" | "Y" | "Z"
Output Specification
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
9A 50 10B 10 20C 20 5D 30 35E 35 15F 15 5G 5 10H 10 20I 20 25ABC(AA)(AB)(AC)(A(BC))((AB)C)(((((DE)F)G)H)I)(D(E(F(G(HI)))))((D(EF))((GH)I))
Sample Output
000error10000error350015000405004750015125
思路：類似于加減法。用堆棧模擬計算
import java.util.Scanner;import java.util.Stack;public class Main {	public static void main(String[] args) {		Scanner scan = new Scanner(System.in);		int N = scan.nextInt();		Matrix[] m = new Matrix[26];		for(int i=0;i<N;i++){			char c = scan.next().charAt(0);			int a = scan.nextInt();			int b = scan.nextInt();			m[c-'A'] = new Matrix(a, b);		}		while(scan.hasNext()){			String str = scan.next();			Stack<Matrix> stack = new Stack<Matrix>();			boolean error = false;			int ans = 0;			for(int i=0;i<str.length();i++){				char c = str.charAt(i);				if(c>='A'&&c<='Z'){					stack.push(m[c-'A']);				}else if(c==')'){					Matrix m2 = stack.pop();					Matrix m1 = stack.pop();					if(m1.b!=m2.a){						error = true;						break;					}else{						ans += m1.a*m1.b*m2.b;						stack.push(new Matrix(m1.a,m2.b));					}				}			}			System.out.println(error?"error":ans);		}	}		static class Matrix{		int a;		int b;		public Matrix(int a,int b){			this.a = a;			this.b = b;		}	}}