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;
    }
}
*/
import java.util.Stack;
public class Solution {
    TreeNode KthNode(TreeNode root, int k){
        if(root==null||k<=0){
            return null;
        }
        Stack<TreeNode> stk=new Stack<TreeNode>();
        while(!stk.isEmpty()||root!=null){
            if(root!=null){
                stk.push(root);
                root=root.left;
            }else{
                root=stk.pop();
                if(k==1){
                    return root;
                }
                --k;
                root=root.right;
            }
        }
        return null;
    }
}