230. Kth Smallest Element in a BST

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

代码：

/**
 * 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) {
        if(root==null)
            return 0;
        ArrayList<Integer> res = new ArrayList<>();
        getnum(root,res);

        int[] temp = new int[res.size()];
        for(int i=0;i<res.size();i++)
            temp[i] = res.get(i);
        Arrays.sort(temp);
        return temp[k-1];
    }
    public void getnum(TreeNode root, List<Integer>res)
    {
        if(root==null)
            return;
        res.add(root.val);
        getnum(root.left,res);
        getnum(root.right,res);
    }
}