UVA442 Matrix Chain Multiplication【stack】【栈】

 Matrix Chain Multiplication 

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 ( tex2html_wrap_inline28 ), 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

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

题目大意：给你N个矩阵的规模，再给你矩阵链乘的表达书，输出乘法的次数。

思路：利用栈来解表达式，遇到字母时，矩阵入栈，遇到右括号')'出栈并计算

结果，再将结果入栈。

#include<iostream>
#include<algorithm>
#include<stack>
#include<cstdio>
#include<cstring>
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;
    char ch[2];
    for(int i = 0; i < N; i++)
    {
        scanf("%s",ch);
        int k = ch[0] -'A';
        cin >> m[k].a >> m[k].b;
    }

    string expre;
    while(cin >> expre)
    {
        int ans = 0,flag = 1;
        int len = expre.length();
        for(int i = 0; i < len; i++)
        {
            if(isalpha(expre[i]))
            {
                s.push(m[expre[i]-'A']);
            }
            else if(expre[i] == ')')
            {
                Matrix x1 = s.top();
                s.pop();
                Matrix x2 = s.top();
                s.pop();
                if(x1.a != x2.b)
                {
                    flag = 0;
                    break;
                }
                ans += x1.b * x1.a * x2.a;
                s.push(Matrix(x2.a,x1.b));
            }
        }
        if(!flag)
            printf("error\n");
        else
            printf("%d\n",ans);
    }

    return 0;
}