LeetCode - Construct Binary Tree Preorder and Inorder Traversal

Given two integer arrays preorder and inorder where preorder is the preorder traversal of a binary tree and inorder is the inorder traversal of the same tree, construct and return the binary tree.

 

Example 1:

Input: preorder = [3,9,20,15,7], inorder = [9,3,15,20,7]
Output: [3,9,20,null,null,15,7]

Example 2:

Input: preorder = [-1], inorder = [-1]
Output: [-1]

 

Constraints:

• 1 <= preorder.length <= 3000
• inorder.length == preorder.length
• -3000 <= preorder[i], inorder[i] <= 3000
• preorder and inorder consist of unique values.
• Each value of inorder also appears in preorder.
• preorder is guaranteed to be the preorder traversal of the tree.
• inorder is guaranteed to be the inorder traversal of the tree.

 

ChatGPT's Solution:

# 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 buildTree(self, preorder: List[int], inorder: List[int]) -> Optional[TreeNode]:
        if not preorder or not inorder:
            return None

        # The first element of preorder is always the root
        root_val = preorder[0]
        root = TreeNode(root_val)

        # Find the index of the root in inorder list
        root_index = inorder.index(root_val)

        # Elements before root_index in inorder belong to the left subtree
        # Elements after root_index in inorder belong to the right subtree
        root.left = self.buildTree(preorder[1:root_index+1], inorder[:root_index])
        root.right = self.buildTree(preorder[root_index+1:], inorder[root_index+1:])

        return root