LeetCode problem 2: Evaluate Reverse Polish Notation

class Solution {
public:
int evalRPN(vector<string> &tokens) {
stack<int> num;
int a,b,c;
for(vector<string>::iterator it = tokens.begin();it != tokens.end();it++){
if("+" == *it || "*" == *it || "-" == *it || "/" == *it){
a = num.top();
num.pop();
b = num.top();
num.pop();

char str = (*it).at(0);
switch(str){
case '+': c = a + b; break;
case '-': c = b - a; break;
case '*': c = a*b;   break;
case '/': c = b/a;   break;
}
num.push(c);
}
else{
const char *p;
int n;
p = (*it).c_str();
n = atoi(p);
num.push(n);
}
}
a = num.top();
return a;
}
};