It is not hard as long as you are familiar with the feature of preorder and inorder traversal.

```
# Definition for a binary tree node.
# class TreeNode(object):
# def __init__(self, x):
# self.val = x
# self.left = None
# self.right = None
class Solution(object):
def buildTree(self, preorder, inorder):
"""
:type preorder: List[int]
:type inorder: List[int]
:rtype: TreeNode
"""
def construct(preorder, x1, y1, inorder, x2, y2):
if y1-x1 == 0: return None
root = TreeNode(preorder[x1])
# find the root in inorder
i = x2
while i < y2:
if inorder[i] == preorder[x1]: break
i += 1
# partition left subtree and right subtree
root.left = construct(preorder, x1+1, x1+1+(i-x2), inorder, x2, i)
root.right = construct(preorder, x1+1+(i-x2), y1, inorder, i+1, y2)
return root
return construct(preorder, 0, len(preorder), inorder, 0, len(inorder))
```