[河南省ACM省赛-第四届] 表达式求值(nyoj 305)

栈的模拟应用：

#include<iostream>
#include<cstdio>
#include<cstring>
#include<string>
#include<cstdlib>
#include<algorithm>
#include<stack>
using namespace std;

string getPostfixExp(string s)
{
    stack<char> sta;// d n x
    string exp;
    int len = s.size();
    for(int i=0; i<len; i++){
        char ch = s[i];
        if(ch == ‘a‘ || ch == ‘m‘) { //如果是字母
            i += 2;
            sta.push(s[i]);
        } else if(ch == ‘(‘){
            sta.push(ch);
        } else if(ch == ‘)‘){//此时顶一定是（
            sta.pop();//弹走（
            exp += " ";
            exp += sta.top();
            sta.pop();
        } else if(ch != ‘,‘){ //数字
            string num;
            while(s[i] >= ‘0‘ && s[i] <= ‘9‘){
                num += s[i++];
            }
            if(exp[0] != NULL)//后缀表达式的第一个一定是数字,用此来避免首位空格
                exp += " ";
            exp += num;
            i--;
        }
    }
    return exp;
}

int calculate(string post)
{
    stack<int>sta;
    int len = post.size();
    for(int i=0; i<len; i++){
        int ch = post[i];
        if(ch >= ‘a‘) {//如果是运算符
            int a = sta.top(); sta.pop();
            int b = sta.top(); sta.pop();
            if(ch == ‘d‘)
                sta.push(a+b);
            else if(ch == ‘x‘)
                sta.push(max(a, b));
            else if(ch == ‘n‘)
                sta.push(min(a, b));
        } else if(ch != ‘ ‘) {//如果为数字
            int num = 0;
            while(post[i] >= ‘0‘ && post[i] <= ‘9‘)
                num = num*10+post[i++]-‘0‘;
            sta.push(num);
            i--;
        }
    }
    return sta.top();
}
int main()
{
    freopen("d:\\in.txt", "r", stdin);
    string s;
    int t;
    cin>>t;
    while(t--) {
        cin>>s;
        string post = getPostfixExp(s);
        cout<<calculate(post)<<endl;
    }
    return 0;
}