public TreeNode buildTree(Vector<Integer> v ,int sta, int end){
        if(end - sta < 0 )
            return null;
        int mid = (sta + end)/2;
        TreeNode node = new TreeNode(v.get(mid));
        node.left = buildTree(v,sta,mid-1);
        node.right = buildTree(v,mid+1,end);
        return node;
    }