# class TreeNode:
#     def __init__(self, x):
#         self.val = x
#         self.left = None
#         self.right = None

class Solution:
    # @param preorder, a list of integers
    # @param inorder, a list of integers
    # @return a tree node
    def buildTree(self, preorder, inorder):
        if not inorder: return None # inorder is empty
        root = TreeNode(preorder[0])
        rootPos = inorder.index(preorder[0])
        root.left = self.buildTree(preorder[1 : 1 + rootPos], inorder[ : rootPos])
        root.right = self.buildTree(preorder[rootPos + 1 : ], inorder[rootPos + 1 : ])
        return root