def helper(self, nums):
if len(nums) == 0:
return None
pivot = len(nums) // 2
root = TreeNode(nums[pivot])
root.left = self.helper(nums[:pivot])
root.right = self.helper(nums[pivot + 1:])
return root
def buildTree(self, preorder: List[int], inorder: List[int]) -> TreeNode:
if preorder is None or len(preorder) != len(inorder) or len(preorder) == 0:
return None
i = 0
while i < len(inorder) and inorder[i] != preorder[0]:
i += 1
root = TreeNode(preorder[0])
root.left = self.buildTree(preorder[1:i + 1], inorder[0:i])
root.right = self.buildTree(preorder[i + 1:], inorder[i + 1:])
return root
def flatten(self, root: TreeNode) -> None:
"""
Do not return anything, modify root in-place instead.
"""
tail = TreeNode(0)
queue = [root] if root else []
while queue:
node = queue.pop()
if node.right:
queue.append(node.right)
if node.left:
queue.append(node.left)
node.left = None
tail.right = node
tail = tail.right
def generate(self, start, end):
if start > end:
return [None]
if start == end:
return [TreeNode(start)]
result = []
for num in range(start, end + 1):
lefts = self.generate(start, num - 1)
rights = self.generate(num + 1, end)
for left in lefts:
for right in rights:
root = TreeNode(num)
root.left = left
root.right = right
result.append(root)
return result
