101.java

class TreeNode {
    int val;
    TreeNode left;
    TreeNode right;

    TreeNode(int x) {
        val = x;
    }
}

/**
  * 参考网上的解题思路
  */

class Solution {
    public boolean isSymmetric(TreeNode root) {
        if (root == null) {
            return true;
        }
        return isSymmetric(root.left, root.right);
    }

    public boolean isSymmetric(TreeNode left, TreeNode right) {
        if (left == null && right == null) {
            return true;
        }

        if (left == null && right != null) {
            return false;
        }

        if (left != null && right == null) {
            return false;
        }

        if (left.val != right.val) {
            return false;
        } else {
            return isSymmetric(left.left, right.right) && isSymmetric(left.right, right.left);
        }
    }

    public static void main(String[] args) {
        TreeNode root = new TreeNode(1);
        TreeNode left = new TreeNode(2);
        TreeNode right = new TreeNode(2);
        root.left = left;
        root.right = right;
        System.out.println(new Solution().isSymmetric(root));
    }
}