150. Evaluate Reverse Polish Notation

• Difficulty: Medium

• Topics: Stack

• Similar Questions:

  • Basic Calculator

  • Expression Add Operators

Problem:

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

Solutions:

class Solution {
public:
    int evalRPN(vector<string>& tokens) {
        stack<int> nums;
        for (auto& token : tokens) {
            if (token == "+" || token == "-" || token == "*" || token == "/") {
                int op1 = nums.top(); nums.pop();
                int op2 = nums.top(); nums.pop();
                int ret;
                switch(token[0]) {
                    case '+': 
                        ret = op1 + op2;
                        break;
                    case '-':
                        ret = op2 - op1; // take care of the order
                        break;
                    case '*':
                        ret = op1 * op2;
                        break;
                    case '/':
                        ret = op2 / op1; // take care of the order
                        break;
                }
                nums.push(ret);
            } else {
                nums.push(stoi(token));
            }
        }

        return nums.top();
    }
};