101. Symmetric Tree

和same tree很像，判断root为空的情况，并且左右子树相反。

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

public boolean dfs(TreeNode left, TreeNode right) {
    if (left == null || right == null) return left == right;

    return (left.val == right.val) && 
            dfs(left.left, right.right) && dfs(left.right, right.left);
}