Evaluate Reverse Polish Notation.cpp

/*
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.

Note:
Division between two integers should truncate toward zero.
The given RPN expression is always valid. That means the expression would always evaluate to a result and there won't be any divide by zero operation.

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

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

Example 3:
Input: ["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> s;
    for (auto &token : tokens)
    {
      if (token == "+")
      {
        int n1 = s.top();
        s.pop();
        int n2 = s.top();
        s.pop();
        s.push(n1 + n2);
      }
      else if (token == "-")
      {
        int n1 = s.top();
        s.pop();
        int n2 = s.top();
        s.pop();
        s.push(n2 - n1);
      }
      else if (token == "*")
      {
        int n1 = s.top();
        s.pop();
        int n2 = s.top();
        s.pop();
        s.push(n1 * n2);
      }
      else if (token == "/")
      {
        int n1 = s.top();
        s.pop();
        int n2 = s.top();
        s.pop();
        s.push(n2 / n1);
      }
      else
        s.push(stoi(token));
    }
    return s.top();
  }
};