leetcode/problems/0150-cpp/main.cpp at main · emfomy/leetcode · GitHub

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// Source: https://leetcode.com/problems/evaluate-reverse-polish-notation
// Title: Evaluate Reverse Polish Notation
// Difficulty: Medium
// Author: Mu Yang <http://muyang.pro>

////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////
// You are given an array of strings `tokens` that represents an arithmetic expression in a **Reverse Polish Notation** (http://en.wikipedia.org/wiki/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 <= 10^4`
// - `tokens[i]` is either an operator: `"+"`, `"-"`, `"*"`, or `"/"`, or an integer in the range `[-200, 200]`.
//
////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////

#include <stack>
#include <string>

using namespace std;

// Use stack
class Solution {
 public:
  int evalRPN(vector<string>& tokens) {
    auto nums = stack<int>();

    for (auto& token : tokens) {
      if (token == "+") {
        auto num2 = nums.top();
        nums.pop();
        auto num1 = nums.top();
        nums.pop();
        nums.push(num1 + num2);
      } else if (token == "-") {
        auto num2 = nums.top();
        nums.pop();
        auto num1 = nums.top();
        nums.pop();
        nums.push(num1 - num2);
      } else if (token == "*") {
        auto num2 = nums.top();
        nums.pop();
        auto num1 = nums.top();
        nums.pop();
        nums.push(num1 * num2);
      } else if (token == "/") {
        auto num2 = nums.top();
        nums.pop();
        auto num1 = nums.top();
        nums.pop();
        nums.push(num1 / num2);
      } else {
        auto num = stoi(token);
        nums.push(num);
      }
    }
    return nums.top();
  }
};