二叉搜索树的第k个结点

解题思路：二叉搜索树的中序遍历是可将结点升序排列的，可中序遍历后输出第k个结点。

import java.util.*;
/*
public class TreeNode {
    int val = 0;
    TreeNode left = null;
    TreeNode right = null;

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

    }

}
*/
public class Solution {
    public TreeNode KthNode(TreeNode pRoot, int k)
    {
        ArrayList<TreeNode> sort=new ArrayList<TreeNode>();
        TreeNode node=null;
        if(k==0){
            return node;
        }
        midOrder(pRoot,sort);
        if(k>sort.size()){
            return node;
        }
        return sort.get(k-1);
    }
    public void midOrder(TreeNode root,ArrayList<TreeNode> sort){
        if(root==null){
            return;
        }
        midOrder(root.left,sort);
        sort.add(root);
        midOrder(root.right,sort);
    }


}