150. Evaluate Reverse Polish Notation

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# 題目敘述

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

# 解題思路

# Solution

import java.util.*;

class Solution {

    public int evalRPN(String[] tokens) {

        ArrayList<Integer> arr = new ArrayList<>();

        for(int i = 0; i < tokens.length; i++){

            if(tokens[i].equals("+") || tokens[i].equals("-") || tokens[i].equals("*") || tokens[i].equals("/")){

                int temp = math(arr.get(arr.size() - 2),arr.get(arr.size() - 1),tokens[i]);

                arr.remove(arr.size() - 1);

                arr.remove(arr.size() - 1);

                arr.add(temp);

            }

            else{

                arr.add(Integer.parseInt(tokens[i]));

            }

        }

        return arr.get(0);

    }

    public int math(int a, int b, String s) {

        switch (s) {

            case "+":

                return a + b;

            case "-":

                return a - b;

            case "*":

                return a * b;

            case "/":

                return a / b;

        }

        return 0;

    }

}

#include <string>

#include <vector>

#include <iostream>

using namespace std;

class Solution {

 public:

  int evalRPN(vector<string>& tokens) {

    vector<int> arr;

    for (int i = 0; i < tokens.size(); i++) {

      if (tokens[i] == "+" || tokens[i] == "-" || tokens[i] == "*" ||

          tokens[i] == "/") {

        int tmp = math(arr[arr.size() - 2], arr[arr.size() - 1], tokens[i]);

        arr.pop_back();

        arr.pop_back();

        arr.push_back(tmp);

      } else {

        arr.push_back(stoi(tokens[i]));

      }

    }

    return arr[0];

  }

  int math(int a, int b, string s) {

    switch (s[0]) {

      case '+':

        return a + b;

      case '-':

        return a - b;

      case '*':

        return a * b;

      case '/':

        return a / b;

    }

    return 0;

  }

};