> ## Documentation Index
> Fetch the complete documentation index at: https://leetcode-py.wisl.dev/llms.txt
> Use this file to discover all available pages before exploring further.

> ## Agent Instructions
> leetcode-py is a Python LeetCode practice environment generator with one CLI: lcpy. It is not a service or platform.
> Each problem is a directory under leetcode/ with README.md, solution.py, test_solution.py, helpers.py, and playground.ipynb. lcpy gen creates them from JSON templates bundled with the package.
> Examples are backed by tests; copy them verbatim.

# Evaluate Reverse Polish Notation

> Tested Python solution for LeetCode 150 with 13 pytest cases. Generate a practice environment with lcpy.

LeetCode 150, Medium. Topics: Array, Math, Stack. [View on LeetCode](https://leetcode.com/problems/evaluate-reverse-polish-notation/description/).

Generate this problem as a practice environment: tested reference solution, 13 [parametrized pytest cases](/practice/testing), and a playground notebook:

```bash theme={"theme":{"light":"github-light","dark":"github-dark"}}
lcpy gen -n 150   # by problem number
lcpy gen -s evaluate_reverse_polish_notation   # by problem name
```

## Problem

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.

### Examples

```
Input: tokens = ["2","1","+","3","*"]
Output: 9
```

**Explanation:** ((2 + 1) \* 3) = 9

```
Input: tokens = ["4","13","5","/","+"]
Output: 6
```

**Explanation:** (4 + (13 / 5)) = 6

```
Input: tokens = ["10","6","9","3","+","-11","*","/","*","17","+","5","+"]
Output: 22
```

**Explanation:** ((10 \* (6 / ((9 + 3) \* -11))) + 17) + 5 = 22

### Constraints

* `1 <= tokens.length <= 10^4`
* `tokens[i]` is either an operator: `"+"`, `"-"`, `"*"`, or `"/"`, or an integer in the range `[-200, 200]`.

## Solution

Reference implementation from [solution.py on GitHub](https://github.com/wislertt/leetcode-py/blob/main/leetcode/evaluate_reverse_polish_notation/solution.py), full suite in [test\_solution.py](https://github.com/wislertt/leetcode-py/blob/main/leetcode/evaluate_reverse_polish_notation/test_solution.py):

```python theme={"theme":{"light":"github-light","dark":"github-dark"}}
class Solution:
    # Time: O(n)
    # Space: O(n)
    def eval_rpn(self, tokens: list[str]) -> int:
        stack: list[int] = []
        ops = {
            "+": lambda a, b: a + b,
            "-": lambda a, b: a - b,
            "*": lambda a, b: a * b,
            "/": lambda a, b: int(a / b),
        }

        for token in tokens:
            if token in ops:
                b, a = stack.pop(), stack.pop()
                stack.append(ops[token](a, b))
            else:
                stack.append(int(token))

        return stack[0]
```

## Complexity

| Time | Space |
| ---- | ----- |
| O(n) | O(n)  |

## Tags

[Grind 75](/catalog/grind-75), [Grind](/catalog/grind), [NeetCode 150](/catalog/neetcode-150), [NeetCode 250](/catalog/neetcode-250), [NeetCode All](/catalog/neetcode).