0101. 对称二叉树 (*)

1, 题目

给定一个二叉树，检查它是否是镜像对称的。

例如，二叉树 [1,2,2,3,4,4,3] 是对称的。

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

但是下面这个 [1,2,2,null,3,null,3] 则不是镜像对称的:

    1
  / \
 2   2
  \   \
  3    3

说明:
如果你可以运用递归和迭代两种方法解决这个问题，会很加分。

来源：力扣（LeetCode）
链接：https://leetcode-cn.com/problems/symmetric-tree
著作权归领扣网络所有。商业转载请联系官方授权，非商业转载请注明出处。

2, 算法

#scala
object Solution {
    def isSymmetric(root: TreeNode): Boolean = {
        if (root == null) {
            return true
        }
        isSymmetricTree(root.left, root.right)
    }
    def isSymmetricTree(left: TreeNode, right: TreeNode): Boolean = {
        if (left == null && right == null) {
            true
        } else if (left == null || right == null) {
            false
        } else if (left.value != right.value) {
            false
        } else {
            isSymmetricTree(left.left, right.right) && isSymmetricTree(left.right, right.left)
        }
    }
}

#python
class Solution:
    def isSymmetric(self, root: TreeNode) -> bool:
        if root:
            return self.isSymmetricTree(root.left, root.right)
        else:
            return True
    def isSymmetricTree(self, left: TreeNode, right: TreeNode) -> bool:
        if left is None and right is None:
            return True
        elif left and right:
            if left.val == right.val:
                return self.isSymmetricTree(left.left, right.right) and self.isSymmetricTree(left.right, right.left)
            else:
                return False
        else:
            return False