[LeetCode]230. Kth Smallest Element in a BST

Given a binary search tree, write a function kthSmallest to find the kth smallest element in it.

Note: 

You may assume k is always valid, 1 ? k ? BST's total elements.

思路：从最小的开始进行从1编号，直到等于k停止并返回值。

使用一个栈，先从root一直选择左节点到最后为止，都压入栈中，此时栈顶就为最小的节点，表为1，随后开始弹栈

弹栈后先进行判断，如果有右节点，则说明在此节点和此时栈顶之前还有介于这两个值的节点没有遍历，然后把这个右节点的选择左节点到最后压入栈中，最后再将编号加一，进行下一个节点递归

/** * Definition for a binary tree node. * public class TreeNode { *     int val; *     TreeNode left; *     TreeNode right; *     TreeNode(int x) { val = x; } * } */public class Solution {    public int kthSmallest(TreeNode root, int k) {        Stack<TreeNode> stack=new Stack<TreeNode>();        TreeNode h=root;        while(h!=null){        stack.push(h);        h=h.left;        }        return fun(stack.pop(),1,k,stack);    }public int fun(TreeNode root,int n,int k,Stack<TreeNode> stack){if(n==k){return root.val;}if(root.right!=null){TreeNode h=root.right;        while(h!=null){        stack.push(h);        h=h.left;        }}        return fun(stack.pop(),n+1,k,stack);}}