二叉搜索树的第k个结点

给定一颗二叉搜索树，请找出其中的第k大的结点。例如， 5 / \ 3 7 /\ /\ 2 4 6 8 中，按结点数值大小顺序第三个结点的值为4。

if(pRoot == null) return null;        //由于是搜索二叉树，取出所有值        ArrayList<TreeNode> allList = new ArrayList<TreeNode>();        ArrayList<TreeNode> layerList = new ArrayList<TreeNode>();        layerList.add(pRoot);        while(layerList.size() != 0){            ArrayList<TreeNode> tempList = new ArrayList<TreeNode>();            for(TreeNode treeNode : layerList){                allList.add(treeNode);                if(treeNode.left != null) tempList.add(treeNode.left);                if(treeNode.right != null) tempList.add(treeNode.right);            }            layerList = tempList;        }        if(k > allList.size() || k <= 0) return null;        //取值排序        for(int i = 0;i < allList.size() - 1;i ++){            for(int j = 0;j < allList.size() - i - 1;j ++){                if(allList.get(j).val > allList.get(j + 1).val){                    TreeNode tempNode = allList.get(j);                    allList.set(j, allList.get(j + 1));                    allList.set(j + 1, tempNode);                }            }        }        //list已经排序结束        //打印        for(TreeNode treeNode : allList){            System.out.println(treeNode.val);        }        return allList.get(k - 1);