代码随想录算法训练营第十四天 | 二叉树part02
226 翻转二叉树 （优先掌握递归）
递归法：前序遍历
if not root :
return None
root.left , root.right = root.right , root.left
self.invertTree(root.left)
self.invertTree(root.right)
return root
迭代法：前序遍历
if not root :
return None
stack = [root]
while stack:
node = stack.pop()
node.left,node.right = node.right,node.left
if node.left:
stack.append(node.left)
if node.right:
stack.append(node.right)
return root
101 对称二叉树 （优先掌握递归）
正是因为要遍历两棵树而且要比较内侧和外侧节点，所以准确的来说是一个树的遍历顺序是左右中，一个树的遍历顺序是右左中。
def isSymmetric(self, root: Optional[TreeNode]) -> bool:
if not root:
return True
def isMirror(left: Optional[TreeNode], right: Optional[TreeNode]) -> bool:
if not left and not right:
return True
if not left or not right:
return False
return (left.val == right.val) and \
isMirror(left.left, right.right) and \
isMirror(left.right, right.left)
return isMirror(root.left, root.right)
104 二叉树的最大深度 （优先掌握递归）
递归法：
def maxdepth(self, root: treenode) -> int:
return self.getdepth(root)
def getdepth(self, node):
if not node:
return 0
leftheight = self.getdepth(node.left) #左
rightheight = self.getdepth(node.right) #右
height = 1 + max(leftheight, rightheight) #中
return height
简化代码：
def maxdepth(self, root: treenode) -> int:
if not root:
return 0
return 1 + max(self.maxdepth(root.left), self.maxdepth(root.right))
111 二叉树的最小深度 （优先掌握递归）
递归法版本一：
def getDepth(self, node):
if node is None:
return 0
leftDepth = self.getDepth(node.left) # 左
rightDepth = self.getDepth(node.right) # 右
# 当一个左子树为空，右不为空，这时并不是最低点
if node.left is None and node.right is not None:
return 1 + rightDepth
# 当一个右子树为空，左不为空，这时并不是最低点
if node.left is not None and node.right is None:
return 1 + leftDepth
result = 1 + min(leftDepth, rightDepth)
return result
def minDepth(self, root):
return self.getDepth(root)
版本二：
class Solution:
def minDepth(self, root):
if root is None:
return 0
if root.left is None and root.right is not None:
return 1 + self.minDepth(root.right)
if root.left is not None and root.right is None:
return 1 + self.minDepth(root.left)
return 1 + min(self.minDepth(root.left), self.minDepth(root.right))
版本三：
class Solution:
def __init__(self):
self.result = float('inf')
def getDepth(self, node, depth):
if node is None:
return
if node.left is None and node.right is None:
self.result = min(self.result, depth)
if node.left:
self.getDepth(node.left, depth + 1)
if node.right:
self.getDepth(node.right, depth + 1)
def minDepth(self, root):
if root is None:
return 0
self.getDepth(root, 1)
return self.result
