剑指 Offer 28. 对称的二叉树

递归

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

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