Evaluate Reverse Polish Notation

int evalRPN(vector<string>& tokens) {

stack<int> s;

int a,b;

for(auto t : tokens){

if(isOperator(t)){

b=s.top();

s.pop();

a=s.top();//ab顺序反了！！

s.pop();

switch (t[0]){//必须是char

case '+':

a+=b;

break;

case '-':

a-=b;

break;

case '*':

a*=b;

break;

case '/':

a/=b;

break;

}

s.push(a);

}

else{

s.push(stoi(t));

}

}

return s.top();

}

bool isOperator(const string &o){

return o.size()==1&&string("+-*/").find(o)!=string::npos;

}