# Solving Leetcode Interviews in Seconds with AI: Evaluate Reverse Polish Notation


	# Introduction
	In this blog post, we will explore how to solve the LeetCode problem "150" using AI. LeetCode is a popular platform for preparing for coding interviews, and with the help of AI tools like [Chatmagic](https://www.chatmagic.app), we can generate solutions quickly and efficiently - helping you pass the interviews and get the job offer without having to study for months.

	# Problem Statement
	> 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].  

	# Explanation
	*   **Stack-Based Evaluation:** Use a stack to store operands. When an operator is encountered, pop the necessary number of operands from the stack, perform the operation, and push the result back onto the stack.
*   **Operator Handling:** Implement a mapping or conditional logic to correctly apply the appropriate arithmetic operation based on the operator token.
*   **Integer Conversion:** Convert the string tokens representing integers into actual integer values for computation.

*   **Runtime Complexity:** O(n), where n is the number of tokens. **Storage Complexity:** O(n) in the worst case (e.g., all operands).

	
	# Code
	```python
	class Solution:
    def evalRPN(self, tokens: list[str]) -> int:
        stack = []
        operators = {
            "+": lambda x, y: x + y,
            "-": lambda x, y: x - y,
            "*": lambda x, y: x * y,
            "/": lambda x, y: int(x / y),  # Truncate towards zero
        }

        for token in tokens:
            if token in operators:
                operand2 = stack.pop()
                operand1 = stack.pop()
                result = operators[token](operand1, operand2)
                stack.append(result)
            else:
                stack.append(int(token))

        return stack[0]
	```