150 - Evaluate Reverse Polish Notation

#medium

You are given an array of strings tokens that represents an arithmetic expression in a Reverse Polish Notation.

Evaluate the expression. Return an integer that represents the value of the expression.

Note that:

• The valid operators are '+', '-', '*', and '/'.
• Each operand may be an integer or another expression.
• The division between two integers always truncates toward zero.
• There will not be any division by zero.
• The input represents a valid arithmetic expression in a reverse polish notation.
• The answer and all the intermediate calculations can be represented in a 32-bit integer.

Example 1:
Input: tokens = ["2","1","+","3","*"]
Output: 9
Explanation: ((2 + 1) * 3) = 9

Example 2:
Input: tokens = ["4","13","5","/","+"]
Output: 6
Explanation: (4 + (13 / 5)) = 6

Example 3:
Input: tokens = ["10","6","9","3","+","-11","","/","","17","+","5","+"]
Output: 22
Explanation: ((10 * (6 / ((9 + 3) * -11))) + 17) + 5
= ((10 * (6 / (12 * -11))) + 17) + 5
= ((10 * (6 / -132)) + 17) + 5
= ((10 * 0) + 17) + 5
= (0 + 17) + 5
= 17 + 5
= 22

class Solution {
public:
    int evalRPN(vector<string>& tokens) {
        stack<int>st;
        int num1, num2;
        for ( int i = 0; i< tokens.size(); i++ ) {
            if ( tokens[i] == "+" || tokens[i] == "-" || tokens[i] == "*" 
                 || tokens[i] == "/" ) {
                num2 = st.top();
                st.pop();
                num1 = st.top();
                st.pop();
                if ( tokens[i] == "+" ) num1 += num2;
                if ( tokens[i] == "-" ) num1 -= num2;
                if ( tokens[i] == "*" ) num1 *= num2;
                if ( tokens[i] == "/" ) num1 /= num2;
                st.push(num1);
            }
            else {
                st.push( stoi(tokens[i]) );
            }
        }

        return st.top();
    }
};