算术表达式求值：含乘方，单目减，运算量为变量或实数。

#include<bits/stdc++.h>
using namespace std;
string in;
stack<char> op;
stack<double> num;
int cmp(char a,char b){
    map<char,int> prior;
    prior['\n']=0;
    prior['+']=prior['-']=1;
    prior['*']=prior['/']=2;
    prior['^']=3;
    if(a=='('&&b==')') return 0;
    if(a=='('||b=='(') return -1;
    if(a==')'||b==')'||prior[a]==prior[b]) return 1;
    return (prior[a]-prior[b])>0?1:-1;
}
int main(){
    while(1){
    const int x=2;
    double tn;
    char tc,tp='\n';
    op.push('\n');
    tc=cin.get();
    while(tc!='\n'||op.top()!='\n'){
        if(isalnum(tc)){
            if(tc=='x') tn=x;
            else{
                cin.putback(tc);
                cin>>tn;
            }
            num.push(tn);
            tp=tc;
            tc=cin.get();
        }
        else{
            if(tc=='-'&&!isalnum(tp)) num.push(0);
            switch(cmp(op.top(),tc)){
                case -1:
                    op.push(tc);tp=tc;tc=cin.get();break;
                case 0:
                    op.pop();tp=tc;tc=cin.get();break;
                case 1:
                        char c=op.top();op.pop();
                        double b=num.top();num.pop();
                        double a=num.top();num.pop();
                        switch(c){
                            case '+': num.push(a+b);break;
                            case '-': num.push(a-b);break;
                            case '*': num.push(a*b);break;
                            case '/': num.push(a/b);break;
                            case '^': num.push(pow(a,b));break;
                        }
                        break;
            }
        }
    }
    cout<<num.top()<<endl;
    }
    return 0;
}