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1050. Construct Binary Search Tree from Preorder Traversal

My accepted Python solution to LeetCode problem 1050, Construct Binary Search Tree from Preorder Traversal, running in 1ms.

  • Difficulty: Medium
  • Python
  • Runtime 1ms
  • Memory 17.2MB

Read the problem on LeetCode View on GitHub

The problem statement is LeetCode’s and stays on their site. What follows is my accepted solution.

Python

Accepted on LeetCode — runtime 1ms, memory 17.2MB, accepted 2026-01-02.

python
# Definition for a binary tree node.
# class TreeNode:
#     def __init__(self, val=0, left=None, right=None):
#         self.val = val
#         self.left = left
#         self.right = right
class Solution:
    def bstFromPreorder(self, preorder: List[int]) -> Optional[TreeNode]:
        if not preorder:
            return None
        
        root = TreeNode(preorder[0])
        stack = [root]
        
        for i in range(1, len(preorder)):
            node = TreeNode(preorder[i])
            if preorder[i] < stack[-1].val:
                stack[-1].left = node
            else:
                parent = None
                while stack and preorder[i] > stack[-1].val:
                    parent = stack.pop()
                parent.right = node
            stack.append(node)
        
        return root

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