力扣572 另一棵树的子树

思路：采用先序遍历的思想遍历二叉树。 

public boolean isSubtree(TreeNode root, TreeNode subRoot) {
        if(root==null) return false;
        return helper(root,subRoot)||isSubtree(root.left,subRoot)||isSubtree(root.right,subRoot);
    }
 
    private boolean helper(TreeNode root,TreeNode subRoot){
        if(root==null&&subRoot==null) return true;
        if(root==null||subRoot==null) return false;
        if(root.val!=subRoot.val) return false;
        return helper(root.left,subRoot.left)&&helper(root.right,subRoot.right);
    }