输入两棵二叉树A，B，判断B是不是A的子结构。（ps：我们约定空树不是任意一个树的子结构）

/**
public class TreeNode {
    int val = 0;
    TreeNode left = null;
    TreeNode right = null;

    public TreeNode(int val) {
        this.val = val;

    }

}
*/
public class Solution{
    public boolean HasSubtree(TreeNode root1,TreeNode root2) {
        if(root2==null) return false;
        if(root1==null && root2!=null) return false;      
        boolean flag = false;
        flag = isSubTree(root1,root2);
        if(!flag){
            flag = HasSubtree(root1.left, root2);
            if(!flag){
                flag = HasSubtree(root1.right, root2);
            }
        }
        return flag;
    }
       
    private boolean isSubTree(TreeNode root1, TreeNode root2) {
        if(root2==null) return true;
        if(root1==null && root2!=null) return false;      
        if(root1.val==root2.val){
            return isSubTree(root1.left, root2.left) && isSubTree(root1.right, root2.right);
        }else{
        return false;
        }
    }
}