深度遍历

递归版本

# Definition for a binary tree node.
class TreeNode:def __init__(self, val=0, left=None, right=None):self.val = valself.left = leftself.right = rightclass Solution:# 前序遍历def preorderTraversal(self, root: TreeNode) -> list[int]:if not root:return []left = self.preorderTraversal(root.left)right = self.preorderTraversal(root.right)return [root.val] + left + right# 中序遍历def inorderTraversal(self, root: TreeNode) -> list[int]:if not root:return []left = self.preorderTraversal(root.left)right = self.preorderTraversal(root.right)return left + [root.val] + right# 后序遍历def postorderTraversal(self, root: TreeNode) -> list[int]:if not root:return []left = self.preorderTraversal(root.left)right = self.preorderTraversal(root.right)return left + right + [root.val]

非递归版本

# Definition for a binary tree node.
class TreeNode:def __init__(self, val=0, left=None, right=None):self.val = valself.left = leftself.right = rightclass Solution:# 前序遍历def preorderTraversal(self, root: TreeNode) -> list[int]:# 根结点为空则返回空列表if not root:return []stack = [root]result = []while stack:node = stack.pop()# 中结点先处理result.append(node.val)# 右孩子先入栈if node.right:stack.append(node.right)# 左孩子后入栈if node.left:stack.append(node.left)return result# 中序遍历def inorderTraversal(self, root: TreeNode) -> list[int]:if not root:return []stack = []  # 不能提前将root结点加入stack中result = []cur = rootwhile cur or stack:# 先迭代访问最底层的左子树结点if cur:stack.append(cur)cur = cur.left# 到达最左结点后处理栈顶结点else:cur = stack.pop()result.append(cur.val)# 取栈顶元素右结点cur = cur.rightreturn result# 后序遍历def postorderTraversal(self, root: TreeNode) -> list[int]:if not root:return []stack = [root]result = []while stack:node = stack.pop()# 中结点先处理result.append(node.val)# 左孩子先入栈if node.left:stack.append(node.left)# 右孩子后入栈if node.right:stack.append(node.right)# 将最终的数组翻转return result[::-1]

非递归版本（代码统一）

# Definition for a binary tree node.
class TreeNode:def __init__(self, val=0, left=None, right=None):self.val = valself.left = leftself.right = rightclass Solution:# 前序遍历def preorderTraversal(self, root: TreeNode) -> list[int]:result = []st = []if root:st.append(root)while st:node = st.pop()if node != None:if node.right:  # 右st.append(node.right)if node.left:  # 左st.append(node.left)st.append(node)  # 中st.append(None)else:node = st.pop()result.append(node.val)return result# 中序遍历def inorderTraversal(self, root: TreeNode) -> list[int]:result = []st = []if root:st.append(root)while st:node = st.pop()if node != None:if node.right:  # 添加右节点（空节点不入栈）st.append(node.right)st.append(node)  # 添加中节点st.append(None)  # 中节点访问过，但是还没有处理，加入空节点做为标记。if node.left:  # 添加左节点（空节点不入栈）st.append(node.left)else:  # 只有遇到空节点的时候，才将下一个节点放进结果集node = st.pop()  # 重新取出栈中元素result.append(node.val)  # 加入到结果集return result# 后序遍历def postorderTraversal(self, root: TreeNode) -> list[int]:result = []st = []if root:st.append(root)while st:node = st.pop()if node != None:st.append(node)  # 中st.append(None)if node.right:  # 右st.append(node.right)if node.left:  # 左st.append(node.left)else:node = st.pop()result.append(node.val)return result

层次遍历

from collections import dequeclass TreeNode:def __init__(self, val=0, left=None, right=None):self.val = valself.left = leftself.right = rightclass Solution:def levelOrder(self, root: TreeNode) -> list[int]:if not root:return []queue = deque([root])result = []while queue:node = queue.popleft()result.append(node.val)if node.left:queue.append(node.left)if node.right:queue.append(node.right)return result