#150. Evaluate Reverse Polish Notation

• Solved

Description

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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

Constraints:

• 1 <= tokens.length <= 104
• tokens[i] is either an operator: “+”, “-“, “*”, or “/”, or an integer in the range [-200, 200].

My Solution = Best Solution

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public class Solution {
    public int EvalRPN(string[] tokens) {
        Stack<int> expression = new Stack<int>();

        for(int i = 0; i < tokens.Length; i++) {
            if(tokens[i] == "+") {
                expression.Push(expression.Pop() + expression.Pop());
            }else if(tokens[i] == "-") {
                int first = expression.Pop();
                int second = expression.Pop();
                expression.Push(second - first);
            }else if(tokens[i] == "*") {
                expression.Push(expression.Pop() * expression.Pop());
            }else if(tokens[i] == "/") {
                int first = expression.Pop();
                int second = expression.Pop();
                expression.Push(second / first);
            }else {
                expression.Push(Convert.ToInt32(tokens[i]));
            }
        }

        return expression.Pop();
    }
}

Runtime

12 ms / Beats 56.28%

Memory

43.80 MB / Beats 27.01%

Big O Notation

Time complexity: O(n)
Space complexity: O(n)