101.对称二叉树

思想：递归

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