简单来说，就是弄两个栈，判断执行：

上代码： 

#include<iostream>
#include<stack>
#include<cstring>
using namespace std;
stack<char>s1,s2;
char now;
int main(){
    string c;
    cin>>c;
    for(int i=0;i<c.length();i++)
	{
        if(c[i]>='0'&&c[i]<='9')
		{
			s2.push(c[i]);
			if(!(c[i+1]>='0'&&c[i+1]<='9'))
				s2.push('.');	
		} 
        else if(c[i]=='+'||c[i]=='-'){
    		while(!s1.empty()&&s1.top()!='(')
    		{
    			s2.push(s1.top());
    			s1.pop();
			}
			s1.push(c[i]);
		}
        else if(c[i]=='*'||c[i]=='/'){
    		while(!s1.empty()&&(s1.top()=='*'||s1.top()=='/'))
    		{
    		    s2.push(s1.top());
    		    s1.pop();
			}
            s1.push(c[i]);
        }
        else if(c[i]=='('){
			s1.push(c[i]);
        }
        else if(c[i]==')'){
			while(s1.top()!='(')
    		{
    			s2.push(s1.top());
    			s1.pop();
			}
			s1.pop();
        }
    }
    while(!s1.empty())
    {
    	s2.push(s1.top());
    	s1.pop();
	}
	while(!s2.empty())
    {
    	s1.push(s2.top());
    	s2.pop();
	}
	while(!s1.empty())
    {
    	cout<<s1.top();
    	s1.pop();
	}
    return 0;
}

因为csp中常考的就是中、后缀的计算，所以先写这些，后续再更