被网上各种写法恶心到了，下面是一个统一写法。
只要了解了先序遍历，中序和后序遍历都懂了。
这里面用到了栈，用栈不断压入根、左孩子，通过pop来回溯父节点，再访问右孩子。
- 先序遍历
"""
Definition of TreeNode:
class TreeNode:
def __init__(self, val):
self.val = val
self.left, self.right = None, None
"""
class Solution:
"""
@param root: The root of binary tree.
@return: Postorder in ArrayList which contains node values.
"""
result = []
def preorderTraversal(self, root):
# write your code here
if root is None:
return []
stack = []
seq = [] #记录先序访问序列
while ((root!=None) | (len(stack)!=0)):
if root!=None:
seq.append(root.val) #先访问根节点
stack.append(root)
root = root.left
else:
root = stack.pop() #回溯至父节点
root = root.right
return seq
- 中序遍历
"""
Definition of TreeNode:
class TreeNode:
def __init__(self, val):
self.val = val
self.left, self.right = None, None
"""
class Solution:
"""
@param root: The root of binary tree.
@return: Postorder in ArrayList which contains node values.
"""
result = []
def inorderTraversal(self, root):
# write your code here
if root is None:
return []
stack = []
seq = []
output = []
while ((root!=None) | (len(stack)!=0)):
if root!=None:
stack.append(root)
root = root.left
else:
root = stack.pop()
seq.append(root.val) # 左孩子先pop出来，再pop根节点
root = root.right
return seq
- 后序遍历
先序：根左右
后续：左右根
即把先序顺序中的 ‘根左右’转换为‘根右左’，然后反过来就变成了‘左右根’。
"""
Definition of TreeNode:
class TreeNode:
def __init__(self, val):
self.val = val
self.left, self.right = None, None
"""
class Solution:
"""
@param root: The root of binary tree.
@return: Postorder in ArrayList which contains node values.
"""
result = []
def postorderTraversal(self, root):
# write your code here
if root is None:
return []
stack = []
seq = []
output = []
while ((root!=None) | (len(stack)!=0)):
if root!=None:
seq.append(root.val)
stack.append(root)
root = root.right # 这从left变成了 right
else:
root = stack.pop()
root = root.left # 这从right变成了 left
while seq: # 后序遍历 是 将先序遍历的反过来
output.append(seq.pop())
return output
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