給出一個二叉搜尋樹，寫出一個方法，求出樹中的第k小的數

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
import java.util.*;
class Solution {

    //中序遍歷-棧
    public int kthSmallest(TreeNode root, int k){
        if(root==null || k==0) return -1;
        Stack<TreeNode> s = new Stack<TreeNode>();
        TreeNode node = root;
        int knum=0;
        s.add(node);
        while(node!=null || !s.isEmpty()){
            //左邊入棧
            while(node!=null){
                s.add(node);
                node=node.left;
            }
            
            //相當於訪問極小點
            node = s.pop();
            knum++;		
            if(knum==k){
                return node.val;
            }
                
            node = node.right;
        }
        
        return -1;
    }
}