[编程题]二叉搜索树的第k个结点

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

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

    }

}
*/
//二叉搜索树的中序遍历是有序的
import java.util.ArrayList;
public class Solution {
    TreeNode KthNode(TreeNode pRoot, int k)
    {
        if(pRoot==null||k<=0){
            return null;
        }
        ArrayList<TreeNode> arr=new ArrayList<TreeNode>();
        inorder(pRoot,arr);
        //当k大于数组的长度，返回为空
        if(k>arr.size()){
            return null;
        }
        return arr.get(k-1);

    }
    //中序遍历二叉树
    ArrayList<TreeNode> inorder(TreeNode pRoot,ArrayList<TreeNode> list){
        if(pRoot==null){
            return null;
        }
        inorder(pRoot.left,list);
        list.add(pRoot);
        inorder(pRoot.right,list);
        return list;
    }
}