[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