leetcode/cpp/0150-evaluate-reverse-polish-notation.cpp at main · neetcode-gh/leetcode · GitHub

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/*
    Evaluate RPN, valid operators: +, -, *, /

    Stack, if num push, if operator apply to top 2 nums

    Time: O(n)
    Space: O(n)
*/

class Solution {
public:
    int evalRPN(vector<string>& tokens) {
        stack<int> stk;

        for (int i = 0; i < tokens.size(); i++) {
            string token = tokens[i];

            if (token.size() > 1 || isdigit(token[0])) {
                stk.push(stoi(token));
                continue;
            }

            int num2 = stk.top();
            stk.pop();
            int num1 = stk.top();
            stk.pop();

            int result = 0;
            if (token == "+") {
                result = num1 + num2;
            } else if (token == "-") {
                result = num1 - num2;
            } else if (token == "*") {
                result = num1 * num2;
            } else {
                result = num1 / num2;
            }
            stk.push(result);
        }

        return stk.top();
    }
};