Evaluate the value of an arithmetic expression in .

Valid operators are +, -, *, /. Each operand may be an integer or another expression.

Some examples:

["2", "1", "+", "3", "*"] -> ((2 + 1) * 3) -> 9  ["4", "13", "5", "/", "+"] -> (4 + (13 / 5)) -> 6

注意：一定要使用后出栈的操作数作为第一操作数，否则，对于没有对称操作的操作符会出错。

 C++实现代码：

#include
      
       #include
       
        #include
        
         #include
         
          #include
          
           using namespace std;class Solution{public:    int evalRPN(vector
           
             &tokens) { stack
            
              operand; int operand1; int operand2; if(tokens.empty()) return 0; int i; for(i=0; i<(int)tokens.size(); i++) { if(tokens[i]=="+") { if(!operand.empty()) { operand1=operand.top(); operand.pop(); } if(!operand.empty()) { operand2=operand.top(); operand.pop(); } operand2+=operand1; operand.push(operand2); } else if(tokens[i]=="-") { if(!operand.empty()) { operand1=operand.top(); operand.pop(); } if(!operand.empty()) { operand2=operand.top(); operand.pop(); } operand2-=operand1; operand.push(operand2); } else if(tokens[i]=="*") { if(!operand.empty()) { operand1=operand.top(); operand.pop(); } if(!operand.empty()) { operand2=operand.top(); operand.pop(); } operand2*=operand1; operand.push(operand2); } else if(tokens[i]=="/") { if(!operand.empty()) { operand1=operand.top(); operand.pop(); } if(!operand.empty()) { operand2=operand.top(); operand.pop(); } operand2/=operand1; operand.push(operand2); } else { operand1=atoi(tokens[i].c_str()); operand.push(operand1); } } return operand.top(); }};int main(){ Solution s; vector
             
               vec={ "4","13","5","/","+"}; cout<
              
               <