150.evaluate-reverse-polish-notation.cpp

/*
 * @lc app=leetcode id=150 lang=cpp
 *
 * [150] Evaluate Reverse Polish Notation
 *
 * https://leetcode.com/problems/evaluate-reverse-polish-notation/description/
 *
 * algorithms
 * Medium (31.33%)
 * Total Accepted:    150K
 * Total Submissions: 478.9K
 * Testcase Example:  '["2","1","+","3","*"]'
 *
 * 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
 *
 *
 */
#include <vector>
#include <string>
#include <stack>
using namespace std;

class Solution {
public:
    int evalRPN(vector<string>& tokens) {
        stack<int> s;
        int first, second, result;
        for (auto& token : tokens) {
            if (token == "+" || token == "-" || token == "*" || token == "/") {
                second = s.top();
                s.pop();
                first = s.top();
                s.pop();
                if (token == "+") {
                    result = first + second;
                } else if (token == "-") {
                    result = first - second;
                } else if (token == "*") {
                    result = first * second;
                } else {
                    result = first / second;
                }
                s.push(result);
            } else {
                s.push(stoi(token.c_str()));
            }
        }
        return s.top();
    }
};