101. 对称二叉树

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

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
class Solution {
    public boolean isSymmetric(TreeNode root) {
        if (root == null) {
            return true;
        }
        return isMirror(root.left, root.right);
    }

    public boolean isMirror(TreeNode leftNode, TreeNode rightNode) {
        if (leftNode == null && rightNode == null) {
            {
                return true;
            }
        } else if ((leftNode != null && rightNode == null) ||
                (leftNode == null && rightNode != null) ||
                leftNode.val != rightNode.val ||
                !isMirror(leftNode.left, rightNode.right) ||
                !isMirror(leftNode.right, rightNode.left)) {
            return false;
        } else {
            return true;
        }
    }
}