LeetCode230.二叉搜索树中第K小的元素

题目来源：

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

题目描述：

代码如下：

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
class Solution {
    public int kthSmallest(TreeNode root, int k) {
        int count = 0;
        Stack<TreeNode> stack = new Stack<>();
        while(root!=null||!stack.empty()) {
        	while(root!=null) {
        		stack.push(root);
        		root=root.left;
        	}
        	root=stack.pop();
        	count++;
        	if(count==k) {
        		return root.val;
        	}
        	root=root.right;
        }
        return 0;
    }
}