101. 对称二叉树.md

class Solution {
    public boolean isSymmetric(TreeNode root) {
        if (root == null) return false;
        return isSymmetric(root, root);
    }

    public boolean isSymmetric(TreeNode t1, TreeNode t2) {
        if (t1 == null && t2 == null) return true;
        if (t1 == null || t2 == null) return false;
        return t1.val == t2.val && isSymmetric(t1.left, t2.right) && isSymmetric(t1.right, t2.left);
    }
}