How do you solve Evaluate Reverse Polish Notation on LeetCode?

Evaluate Reverse Polish Notation (LeetCode #150) 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: Using a stack is essential for evaluating expressions in Reverse Polish Notation.

What is the time complexity of Evaluate Reverse Polish Notation?

The optimal solution for Evaluate Reverse Polish Notation runs in O(n) time and uses O(n) space. This complexity is linear because we process each token exactly once, and the stack operations (push/pop) are O(1).

What is the brute force solution for Evaluate Reverse Polish Notation?

The brute force approach for Evaluate Reverse Polish Notation is Brute Force, with O(n²) time complexity and O(1) space complexity. The brute-force approach evaluates the expression by recursively calculating the result for each operator and its operands. This is straightforward but inefficient, as it recalculates results multiple times. The optimal Optimal Solution approach improves this to O(n).

What algorithm or data structure is used to solve Evaluate Reverse Polish Notation?

Evaluate Reverse Polish Notation uses the following concepts: Array, Math, Stack. The recommended patterns to study are: Stack, Array.

What are the interview tips for Evaluate Reverse Polish Notation?

Clarify the input format and constraints before starting. Explain your thought process as you code to show your understanding. Test your solution with edge cases, like negative numbers or large integers.

What are common mistakes when solving Evaluate Reverse Polish Notation?

Not handling integer division correctly (truncating towards zero). Confusing the order of operands when applying operators.

#150

Evaluate Reverse Polish Notation

Medium

ArrayMathStackStackArray

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 evaluate the expression in a single pass. This approach is efficient because it processes each token exactly once, making it linear in time complexity.

⚙️

Algorithm

5 steps

1Step 1: Initialize an empty stack.
2Step 2: Iterate through each token in the input array.
3Step 3: If the token is a number, push it onto the stack.
4Step 4: If the token is an operator, pop the top two numbers from the stack, apply the operator, and push the result back onto the stack.
5Step 5: At the end, the stack will contain one element, which is the result.

solution.py19 lines

1# Full working Python code
2
3def evalRPN(tokens):
4    stack = []
5    for token in tokens:
6        if token not in ['+', '-', '*', '/']:
7            stack.append(int(token))
8        else:
9            b = stack.pop()
10            a = stack.pop()
11            if token == '+':
12                stack.append(a + b)
13            elif token == '-':
14                stack.append(a - b)
15            elif token == '*':
16                stack.append(a * b)
17            elif token == '/':
18                stack.append(int(a / b))  # Truncate towards zero
19    return stack[0]

ℹ

Complexity note: This complexity is linear because we process each token exactly once, and the stack operations (push/pop) are O(1).

1Using a stack is essential for evaluating expressions in Reverse Polish Notation.
2Operators are applied immediately after their operands are available.

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