[leetcode tree]101. Symmetric Tree
Given a binary tree, check whether it is a mirror of itself (ie, symmetric around its center).
For example, this binary tree [1,2,2,3,4,4,3]
is symmetric:
1
/ \
2 2
/ \ / \
3 4 4 3
But the following [1,2,2,null,3,null,3]
is not:
1
/ \
2 2
\ \
3 3
判断一棵树是否对称
递归:
class Solution(object):
def isSymmetric(self, root):
if not root:
return True
return self.issym(root.left,root.right)
def issym(self,left,right):
if not left and not right:
return True
if not left or not right:
return False
if left.val == right.val:
return self.issym(left.left,right.right) and self.issym(left.right,right.left)
return False
非递归:
递归本质上就是栈,所以。。。
class Solution(object):
def isSymmetric(self, root):
if not root:
return True
stack=[[root.left,root.right]]
while stack:
left,right = stack.pop()
if not left and not right:
continue
if not left or not right:
return False
if left.val == right.val:
stack.append([left.right,right.left])
stack.append([left.left,right.right])
else:
return False
return True
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