199. Binary Tree Right Side View

    /*
     * 199. Binary Tree Right Side View 
     * 11.21 By Mingyang 
     * 在recursive的算法，就是贴着树的右边界往下面走，如果不行往左边偏一个，然后继续往右边偏，利用末尾的个数与层数相等的技巧
     * 其实是一种根右左的算法，很巧妙
     */    
    public List<Integer> ss = new ArrayList<Integer>();
    public List<Integer> rightSideView(TreeNode root) {
        if (root == null)
            return ss;
        rightHelper(root, ss, 0);
        return ss;
    }
    public void rightHelper(TreeNode root, List<Integer> res, int k) {
        if (root == null)
            return;
        if (k == res.size())
            res.add(root.val);
        rightHelper(root.right, res, k + 1);
        rightHelper(root.left, res, k + 1);
    }