Leetcode 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 is symmetric:

    1
   / \
  2   2
 / \ / \
3  4 4  3

 

But the following is not:

    1
   / \
  2   2
   \   \
   3    3

 

Note:
Bonus points if you could solve it both recursively and iteratively.

 循环

class Solution(object):
    def isSymmetric(self, root):
        stack = [root]
        while stack:
            value = [x.val if x else None for x in stack]
            if value[::-1] != value:
                return False
            stack_new = []
            for x in stack:
                if x:
                    stack_new += [x.left,x.right]
            stack = stack_new
        return True

递归

 注意判断顺序

class Solution(object):
    def isSymmetric(self, root):
        return True if not root else self.sym(root.left, root.right)
    
    def sym(self, l, r):
        if not l and not r:
            return True
        if not l or not r:
            return False
        if l.val != r.val:
            return False
        return self.sym(l.left,r.right) and self.sym(l.right,r.left)