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Problem solution · Python

Design an Expression Tree With Evaluate Function

Design an Expression Tree With Evaluate Function: a Python solution using sliding window or two pointers. Learn the idea, check the complexity, and read the full code, with credit to walkccc LeetCode Solutions.

Technique
Sliding window or two pointers
Source
walkccc LeetCode Solutions
Length
60 lines
Start with the idea.

Try the problem first. If you get stuck, read the approach below, then write your own solution. The full code is at the bottom.

Approach

Sliding window or two pointers

For Design an Expression Tree With Evaluate Function, the implementation maintains a moving interval and updates only the information that enters or leaves the window.

  1. Choose the invariant that makes a window valid or useful.
  2. Advance the right boundary and add the new element.
  3. Move the left boundary only as needed while maintaining the invariant and updating the answer.

Code notes

  • 60 lines of Python from the credited upstream file 1628.py.
  • The implementation visibly relies on sequence storage.
  • No explicit loop blocks detected.

Complexity

Confirm that neither pointer moves backwards; if so, the scan is usually linear apart from the window’s data-structure operations.

Check the problem constraints before deciding whether this complexity will pass.

Source

Code and credit

This code comes from walkccc LeetCode Solutions by P.-Y. Chen (walkccc) and is used under the MIT licence.

Open problem ↗Original source ↗
Full codeDesign an Expression Tree With Evaluate Function · PythonPython
Use this to learn the idea, then write your own version.
from abc import ABC, abstractmethod """This is the interface for the expression tree Node.You should not remove it, and you can define some classes to implement it."""  class Node(ABC):  @abstractmethod  # define your fields here  def evaluate(self) -> int:    pass  class ExpNode(Node):  op = {      '+': lambda a, b: a + b,      '-': lambda a, b: a - b,      '*': lambda a, b: a * b,      '/': lambda a, b: int(a / b),  }   def __init__(      self,      val: str,      left: Optional['ExpNode'],      right: Optional['ExpNode'],  ):    self.val = val    self.left = left    self.right = right   def evaluate(self) -> int:    if not self.left and not self.right:      return int(self.val)    return ExpNode.op[self.val](self.left.evaluate(), self.right.evaluate())  """This is the TreeBuilder class.You can treat it as the driver code that takes the postinfix inputand returns the expression tree represnting it as a Node."""  class TreeBuilder(object):  def buildTree(self, postfix: list[str]) -> 'Node':    stack: list[ExpNode | None] = []     for val in postfix:      if val in '+-*/':        right = stack.pop()        left = stack.pop()        stack.append(ExpNode(val, left, right))      else:        stack.append(ExpNode(val, None, None))     return stack.pop()

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