#101对称二叉树

import java.util.*;

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

    TreeNode(int val) {
        this.val = val;
    }
}

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

    private boolean mirror(TreeNode left, TreeNode right) {
        if (left == null || right == null) return left == right;
        return left.val == right.val
                && mirror(left.left, right.right)
                && mirror(left.right, right.left);
    }
}