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 None
        idx=inorder.index(preorder[0])

        root=TreeNode(preorder[0])
        l=self.buildTree(preorder[1:idx+1],inorder[:idx])

        r=self.buildTree(preorder[idx+1:],inorder[idx+1:])

        root.left=l

        root.right=r

        return root