Symmetric Tree-LeetCode#101

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

Note:
Bonus points if you could solve it both recursively and iteratively.
思路: 对称树。比较左节点和右节点的val，以此类推

public class SymmetricTree {
    public boolean isSymmetric(TreeNode root) {
        if (root == null) return true;
        return isSameNode(root.left, root.right);
    }
    boolean isSameNode(TreeNode left, TreeNode right){
        if (left == null && right == null) return true;
        if ((left == null && right != null) || (left != null && right == null) || left.val != right.val) return false;
        return isSameNode(left.left, right.right) && isSameNode(left.right, right.left);
    }
}