UVA 442 Matrix Chain Multiplication

Description

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 (  ), 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

直接類比

language:c++

code:

#include<iostream>#include<stack>#include<map>#include<cstdio>using namespace std;struct node{    int x,y;    node():x(0),y(0) {}    node(int a,int b):x(a),y(b) {};};node multiple(node a,node b,int& ans){    if(a.y!=b.x)        return node(0,0);    else    {        ans+=a.x*a.y*b.y;        return node(a.x,b.y);    }}int main(){    //freopen("in.txt","r",stdin);    int n,x,y,ans;    bool flag;    string exp;    stack<char>stck;    cin>>n;    char ch,cnt;    map<char,node>m;    for(int i=0; i<n; i++)    {        cin>>ch>>x>>y;        m.insert(make_pair(ch,node(x,y)));    }    while(cin>>exp)    {        flag=0;        while(!stck.empty())stck.pop();        ans=0,cnt='1';        map<char,node>mp(m);        for(int i=exp.size()-1; i>-1; i--)        {            if(exp[i]!='(')            {                stck.push(exp[i]);            }            else            {                node top=mp[stck.top()];                stck.pop();                while(stck.top()!=')')                {                    top=multiple(top,mp[stck.top()],ans);                    if(top.x==0)                    {                        flag=1;break;                    }                    stck.pop();                }                if(flag)break;                stck.pop();                stck.push(cnt);                mp.insert(make_pair(cnt++,top));            }        }        if(!flag)        cout<<ans<<endl;        else        cout<<"error"<<endl;    }    return 0;}