[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