pku 2106 Boolean Expressions

類似表達式求值的題目

 

和表達式求值的做法也差不多

 

用兩個棧分別儲存  操作數  和 運算符 

 

運算符的優先級 爲  ! > & > |

 

有個地方需要注意的是，如果是! 與 ! 自己進行比較，  應該當作 ! < !本身來處理，否則出現 !!!F  這種表達式的時候會出現RE

因爲第二個! 準備進棧時，如果此時處理 第一個!，儲存操作數的棧爲空棧，如果此時調用pra.top() 就會出現RE

 

代碼如下：

 

#include <iostream>
#include <stack>
#include <string>
using namespace std;

int  compare(char a,char b);

int main()
{

    string buffer;
    int ca(1);
    while (getline(cin,buffer))
    {

     buffer.push_back('#');  //表達式最後插入一個‘#’，用來確保表達式能處理完
        stack<char> pra;
        stack<char> ope;

        ope.push('#'); //第一個元素爲‘#’  爲假定的另一種括號，優先級最低，用來判定表達式有沒有處理完

        int i(0);
        int len = buffer.size();

        while (i<len)
        {
            if(buffer[i]==' ') {++i;continue;}
            if (buffer[i]=='F'||buffer[i]=='V')
            {
                pra.push(buffer[i]);

            }
            else
            {



                if (compare(ope.top(),buffer[i])<0)
                    ope.push(buffer[i]);
                else if (compare(ope.top(),buffer[i])>0)
                {

                    char a,b;
                    switch (ope.top())
                    {
                    case '!':
                        a=pra.top();
                        pra.pop();
                        if (a=='V') a='F';
                        else a='V';
                        pra.push(a);
                        break;
                    case '&':
                         a=pra.top();
                         pra.pop();
                         b=pra.top();
                         pra.pop();
                         if(a=='F'||b=='F') pra.push('F');
                         else pra.push('V');
                         break;
                    case '|':
                         a=pra.top();
                         pra.pop();
                         b=pra.top();
                         pra.pop();
                         if(a=='V'||b=='V') pra.push('V');
                         else pra.push('F');
                         break;
                    }
                    ope.pop();
                    continue;
                }
                else
                {
                 ope.pop();

                }
            }


            ++i;
        }
      cout<<"Expression "<<ca++<<": ";
     cout<<pra.top()<<endl;
    }

    return 0;
}

int compare(char a,char b)  //比較運算符的優先級，需要注意的是運算符間的優先級關係並不是絕對的
{
    if (b=='(') return -1;
    switch (a)
    {
    case '(':
        if (b==')') return 0;
        return -1;
    case '!':
        if(b!='!')
        return 1;
        return -1;
    case '&':
        if (b=='!') return -1;
        return 1;
    case '|':
        if (b=='!'||b=='&') return -1;
        return 1;
    case '#':
        if (b=='#') return 0;
        return -1;
    }

    return 0;

}