evaluate-reverse-polish-notation

150. Evaluate Reverse Polish Notation

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

Constraints:

• 1 <= tokens.length <= 104
• tokens[i] is either an operator: "+", "-", "*", or "/", or an integer in the range [-200, 200].

class Solution {
public:
    int evalRPN(vector<string>& tokens) {
        stack<int> st;
        for(auto& token : tokens) {
            if(token != "+" && token != "-" && token != "*" && token != "/") {
                st.push(stoi(token));
            } else {
                int num1 = st.top(); st.pop();
                int num2 = st.top(); st.pop();
                if(token == "+") st.push(num2 + num1);
                if(token == "-") st.push(num2 + num1);
                if(token == "*") st.push(num2 * num1);
                if(token == "/") st.push(num2 / num1);
            }
        }
        return st.top();
    }
};

• T: $O(n)$
• S: $O(n)$