前序遍歷
# Definition for a binary tree node.
# class TreeNode:
# def __init__(self, x):
# self.val = x
# self.left = None
# self.right = None
class Solution:
def preorderTraversal(self, root: TreeNode) -> List[int]:
res=[]
stack=[]
'''由於是先遍歷根節點然後左子樹然後右子樹。所以要先訪問根節點,然後用棧先進右子樹。左邊已經遍歷完了,然後讓最後進入的右子樹出棧繼續遍歷'''
while(root or len(stack)):
while(root):
res.append(root.val)
stack.append(root.right)
root=root.left
root=stack.pop()
return res
前序遍歷右子樹入棧
中序遍歷
class Solution:
def inorderTraversal(self, root: TreeNode) -> List[int]:
ans=[]
stack=[]
'''由於是先左再中再右,就應該先讓左子樹不斷入棧 然後出來一個沒有左節點的根節點,然後遍歷右節點。'''
while len(stack)!=0 or root:
while(root):
stack.append(root)
root=root.left
root=stack.pop()
ans.append(root.val)
if root.right:
root=root.right
else:
root=None
return ans
中序中間弄完弄右邊
後序遍歷
# Definition for a binary tree node.
# class TreeNode:
# def __init__(self, x):
# self.val = x
# self.left = None
# self.right = None
class Solution:
def postorderTraversal(self, root: TreeNode) -> List[int]:
stack=[]
res=[]
'''先左子樹,然後右子樹,然後根節點,先把左子樹入棧,然後只要右子樹爲空或者右子樹已經遍歷過,那麼就訪問出棧。然後遍歷右子樹。'''
while(root or len(stack)):
while(root):
stack.append(root)
root=root.left
while (len(stack) and (stack[-1].right==None or stack[-1].right in res)):
res.append(stack.pop())
if len(stack):
root=stack[-1].right
後序遍歷右子樹弄完弄中間
return [i.val for i in res]