leetcode: Evaluate Reverse Polish Notation

Evaluate the value of an arithmetic expression in Reverse Polish Notation.

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

#include <iostream>
#import <vector>
#include <stack>

using namespace std;
class Solution {
public:
    int evalRPN(vector<string> &tokens) {
        stack<int> *num =new stack<int>;
        for(int index=0;index<tokens.size();index++){
            switch (tokens[index][0]){
                case '*': {
                    int a=num->top();
                    num->pop();
                    int b=num->top();
                    num->pop();
                    num->push(b*a);
                    continue;
                }
                case '/': {
                    int a=num->top();
                    num->pop();
                    int b=num->top();
                    num->pop();
                    num->push(b/a);
                    continue;
                }
                case '+': {
                    int a=num->top();
                    num->pop();
                    int b=num->top();
                    num->pop();
                    num->push(b+a);
                    continue;
                }
                case '-': {
                    if(tokens[index].size()==1){
                    int a=num->top();
                    num->pop();
                    int b=num->top();
                    num->pop();
                    num->push(b-a);  }
                    else{
                        num->push(stoi(tokens[index]));
                    }
                    continue;
                }
                default: {
                    num->push(stoi(tokens[index]));
                }

            }
        }
        return num->top();
    }
};


int main() {
    Solution s;

    string n4[] = {"-2","3","+","4","32","-","*"};
    vector<string> a4(&n4[0], &n4[7]);
    cout<<s.evalRPN(a4)<<endl;

    return 0;

}