递归实现 $O(n^2)$

Python 代码

# 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):
        if not preorder: return
        root = TreeNode(preorder[0])
        idx = inorder.index(preorder[0])
        root.left = self.buildTree(preorder[1:1+idx], inorder[:idx])
        root.right = self.buildTree(preorder[1+idx:], inorder[idx+1:])

        return root