# LeetCode 105: Construct Binary Tree From Preorder And Inorder Traversal — Step-by-Step Visual Trace

**Medium** — Binary Tree | Recursion | Array | Tree Construction

## The Problem

Given two arrays representing preorder and inorder traversals of a binary tree, reconstruct and return the original binary tree.

## Approach

Use the preorder array to identify root nodes and the inorder array to determine left and right subtrees. Recursively build the tree by splitting the inorder array at each root's position and processing the corresponding preorder elements.

**Time:** O(n²) · **Space:** O(n)

## Code

```python
class Solution:
    def buildTree(self, preorder: List[int], inorder: List[int]) -> TreeNode:
        if not preorder or not inorder:
            return None

        root_val = preorder.pop(0)
        root = TreeNode(root_val)
        root_index = inorder.index(root_val)

        root.left = self.buildTree(preorder, inorder[:root_index])
        root.right = self.buildTree(preorder, inorder[root_index + 1 :])

        return root
```

## Watch It Run

> **[Open interactive visualization](https://tracelit.dev/app?trace=0105_construct-binary-tree-from-preorder-and-inorder-traversal)**

> **Try it yourself:** Open [TraceLit](https://tracelit.dev/app?trace=0105_construct-binary-tree-from-preorder-and-inorder-traversal) and step through every line.

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