How do you solve Parse Lisp Expression on LeetCode?

Parse Lisp Expression (LeetCode #736) can be solved using Brute Force or Optimal Solution. The optimal approach is Optimal Solution with O(n) time complexity and O(n) space complexity. Key insight: Understanding token parsing is crucial for evaluating nested expressions.

What is the brute force solution for Parse Lisp Expression?

The brute force approach for Parse Lisp Expression is Brute Force, with O(n²) time complexity and O(1) space complexity. The brute-force approach involves parsing the expression recursively without any optimization. We will evaluate each part of the expression separately, which can lead to repeated evaluations and inefficient handling of nested expressions. The optimal Optimal Solution approach improves this to O(n).

What algorithm or data structure is used to solve Parse Lisp Expression?

Parse Lisp Expression uses the following concepts: Hash Table, String, Stack, Recursion. The recommended patterns to study are: Hash Map, Stack.

What are the interview tips for Parse Lisp Expression?

Clarify the expression format before starting. Think about edge cases, such as empty expressions or deeply nested ones. Explain your thought process while coding to demonstrate your understanding.

What are common mistakes when solving Parse Lisp Expression?

Not handling nested expressions correctly. Forgetting to update the variable scope in let expressions.

#736

Parse Lisp Expression

Hard

Hash TableStringStackRecursionHash MapStack

LeetCode ↗

Approaches

Brute ForceOptimal

Complexity Comparison

	Brute Force	Optimal Solution★
Time	O(n²)	O(n)
Space	O(1)	O(n)

💡

Intuition

Time O(n)Space O(n)

The optimal solution uses a stack to manage the current scope of variables and evaluates expressions in a single pass. This avoids redundant evaluations and efficiently handles nested expressions.

⚙️

Algorithm

4 steps

1Step 1: Initialize a stack to keep track of variable scopes.
2Step 2: Parse the expression and identify tokens based on parentheses balance.
3Step 3: For each token, evaluate it based on its type (integer, variable, let, add, mult) using the current scope.
4Step 4: Update the scope for let expressions and return the final evaluated result.

solution.py8 lines

1def evaluate(expression):
2    stack = [{}]
3    def parse(expr):
4        # Token parsing logic here
5        return tokens
6    tokens = parse(expression)
7    # Evaluation logic here
8    return result

ℹ

Complexity note: The optimal solution runs in linear time because each character in the expression is processed once, and the space complexity accounts for the variable scope stack.

1Understanding token parsing is crucial for evaluating nested expressions.
2Managing variable scope with a stack allows for efficient evaluations.

Solutions and explanations are original Tejav content. Problem titles © LeetCode — use the LeetCode button above for the full problem statement.