树的前序(preorder)遍历：
iterative：
用stack，依次放右节点，左节点
python代码：
class Solution:
"""
@param root: A Tree
@return: Preorder in ArrayList which contains node values.
"""
def preorderTraversal(self, root):
# write your code here
res = []
stack = []
if not root:
return res
stack.append(root)
while stack:
node = stack.pop()
res.append(node.val)
if node.right:
stack.append(node.right)
if node.left:
stack.append(node.left)
return res
recursive:
就很简单啦，res添加当前节点，dfs访问左节点，dfs访问右节点。
class Solution:
"""
@param root: A Tree
@return: Preorder in ArrayList which contains node values.
"""
def preorderTraversal(self, root):
# write your code here
res = []
self.dfs(root, res)
return res
def dfs(self, node, res):
if node:
res.append(node.val)
self.dfs(node.left, res)
self.dfs(node.right, res)
return
树的中序遍历(inorder): 左根右
iterative:
同样是用stack，从根开始依次把左边节点往栈里推，直到节点为空，栈pop，放入result list中，再把当前节点设置为pop出的节点的右节点，再执行上述操作。最后直到栈为空
"""
Definition of TreeNode:
class TreeNode:
def __init__(self, val):
self.val = val
self.left, self.right = None, None
"""
class Solution:
"""
@param root: A Tree
@return: Inorder in ArrayList which contains node values.
"""
def inorderTraversal(self, root):
# write your code here
stack = []
res = []
if not root:
return []
cur = root
while cur or stack:
while cur:
stack.append(cur)
cur = cur.left
node = stack.pop()
res.append(node.val)
cur = node.right
return res
recursive:
class Solution:
"""
@param root: A Tree
@return: Inorder in ArrayList which contains node values.
"""
def inorderTraversal(self, root):
# write your code here
res = []
self.dfs(root, res)
return res
def dfs(self, node, res):
if node:
self.dfs(node.left, res)
res.append(node.val)
self.dfs(node.right, res)
树的后序遍历(post order):
iterative:
class Solution:
"""
@param root: A Tree
@return: Postorder in ArrayList which contains node values.
"""
def postorderTraversal(self, root):
# write your code here
res = []
stack = []
if not root:
return res
stack.append(root)
while stack:
node = stack.pop()
res.append(node.val)
if node.left:
stack.append(node.left)
if node.right:
stack.append(node.right)
res.reverse()
return res