LeetCode | Evaluate Reverse Polish Notation

The algorithm for evaluating any postfix expression is fairly straightforward:

While there are input tokens left

• • Read the next token from input.
  • If the token is a value
    • Push it onto the stack.
  • Otherwise, the token is an operator (operator here includes both operators and functions).
    • It is known a priori that the operator takes n arguments.
    • If there are fewer than n values on the stack
      • (Error) The user has not input sufficient values in the expression.
    • Else, Pop the top n values from the stack.
    • Evaluate the operator, with the values as arguments.
    • Push the returned results, if any, back onto the stack.
• If there is only one value in the stack
  • That value is the result of the calculation.
• Otherwise, there are more values in the stack
  • (Error) The user input has too many values.

class Solution {
public:
    int evalRPN(vector<string> &tokens) {
        if (tokens.empty()) return 0;
        
        stack<int> st;
        
        for (int i = 0; i < tokens.size(); ++i) {
            if (tokens[i] == "+" || tokens[i] == "-" || tokens[i] == "*" || tokens[i] == "/") {
                if (st.empty()) return 0;
                int n1 = st.top();
                st.pop();
                if (st.empty()) return 0;
                int n2 = st.top();
                st.pop();
                if (tokens[i] == "+") st.push(n1 + n2);
                else if (tokens[i] == "-") st.push(n2 - n1);
                else if (tokens[i] == "*") st.push(n2 * n1);
                else if (n1 == 0) return 0;
                else st.push(n2 / n1);
            } else {
                st.push(atoi(tokens[i].c_str()));
            }
        }
        
        return st.top();
    }
};