Google算法题：M-求二叉搜索树的第K小的数

http://www.jiuzhang.com/article/Google%202016%E5%B9%B412%E6%9C%88SDE%E7%94%B5%E9%9D%A2%E9%9D%A2%E7%BB%8F/

https://leetcode.com/problems/kth-smallest-element-in-a-bst/description/

给出一个二叉搜索树，写出一个方法，求出树中的第k小的数

/**
 * 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 {

    //中序遍历-栈
    public int kthSmallest(TreeNode root, int k){
        if(root==null || k==0) return -1;
        Stack s = new Stack();
        TreeNode node = root;
        int knum=0;
        s.add(node);
        while(node!=null || !s.isEmpty()){
            //左边入栈
            while(node!=null){
                s.add(node);
                node=node.left;
            }
            
            //相当于访问极小点
            node = s.pop();
            knum++;		
            if(knum==k){
                return node.val;
            }
                
            node = node.right;
        }
        
        return -1;
    }
}