题目：
题解：
class Solution:
def buildTree(self, preorder: List[int], inorder: List[int]) -> TreeNode:
if not preorder:
return None
root = TreeNode(preorder[0])
stack = [root]
inorderIndex = 0
for i in range(1, len(preorder)):
preorderVal = preorder[i]
node = stack[-1]
if node.val != inorder[inorderIndex]:
node.left = TreeNode(preorderVal)
stack.append(node.left)
else:
while stack and stack[-1].val == inorder[inorderIndex]:
node = stack.pop()
inorderIndex += 1
node.right = TreeNode(preorderVal)
stack.append(node.right)
return root