HNUCM-OJ1232算法3-4表达式求值，中缀转后缀表达式求值

HNUCM-OJ 1232 算法3-4 表达式求值

中缀转后缀表达式求值

题目链接：
HNUCM-OJ 1232 算法3-4 表达式求值

解题思路：

这一个题，关键是怎么表现两个运算符的优先级，在网上看到一个大佬的总结，用一个二维数组来表现，运算符之间的优先级，其中θ1为存放运算符的栈（我自己定义的为s2）的栈顶元素，θ2为从表达式中读取的操作符

#include 
#include 
#include 
#include 

using namespace std;

const int cmax=10e5+5;
char s[cmax];
stack<double> s1;
stack<char> s2;
const char priority[7][7]=
{
    {'>','>','<','<','<','>','>'},
    {'>','>','<','<','<','>','>'},
    {'>','>','>','>','<','>','>'},
    {'>','>','>','>','<','>','>'},
    {'<','<','<','<','<','=','0'},
    {'>','>','>','>','0','>','>'},
    {'<','<','<','<','<','0','='},
};
int getIndex(char c)
{
    int index;
    switch(c)
    {
    case'+':
        index=0;
        break;
    case'-':
        index=1;
        break;
    case'*':
        index=2;
        break;
    case'/':
        index=3;
        break;
    case'(':
        index=4;
        break;
    case')':
        index=5;
        break;
    default:
        index=6;
        break;
    }
    return index;
}
char getPriority(char a,char b)
{
    int index1,index2;
    index1=getIndex(a);
    index2=getIndex(b);
    return priority[index1][index2];
}
double calculate(double num1,char c,double num2)
{
    double sum;
    switch(c)
    {
    case'+':
        sum=num2+num1;
        break;
    case'-':
        sum=num2-num1;
        break;
    case'*':
        sum=num2*num1;
        break;
    case'/':
        sum=num2/num1;
        break;
    }
    return sum;
}
double getAnswer(char s[])
{
    s2.push('#');
    int counter=0;
    int len=strlen(s);
    int i=0;
    while(i<len)
    {
        if(s[i]-'0'>=0&&s[i]-'0'<=9)
        {
            if(counter==1)
            {
                double num1=s1.top();
                s1.pop();
                num1=num1*10+(s[i]-'0');
                s1.push(num1);
                i++;
            }
            else
            {
                s1.push(s[i]-'0');
                i++;
                counter++;
            }
        }
        else
        {
            counter=0;
            char c=getPriority(s2.top(),s[i]);
            switch(c)
            {
            case'<':
                s2.push(s[i]);
                i++;
                break;
            case'>':
            {
                char c1=s2.top();
                s2.pop();
                double num2=s1.top();
                s1.pop();
                double num3=s1.top();
                s1.pop();
                double num4=calculate(num2,c1,num3);
                s1.push(num4);
                break;
            }
            case'=':
                s2.pop();
                i++;
                break;
            }
        }
    }
    return s1.top();
}
int main()
{
    while(~scanf("%s",s))
    {
        while(!s1.empty())
            s1.pop();
        while(!s2.empty())
            s2.pop();
        double num=getAnswer(s);
        cout<<num<<endl;
    }
    return 0;
}