leetcode-java-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.

Follow up:
What if the BST is modified (insert/delete operations) often and you need to find
the kth smallest frequently? How would you optimize the kthSmallest routine?
*/
/**
* Definition for a binary tree node.
* public class TreeNode {
*     int val;
*     TreeNode left;
*     TreeNode right;
*     TreeNode(int x) { val = x; }
* }
*/
/*
因为是BST，则中序（左根右）遍历得到list，找list的第k个元素
*/

public class Solution {
public int kthSmallest(TreeNode root, int k) {
if(root == null) {
return 0;
}

List<Integer> list = new ArrayList<Integer>();

list = traversalTree(root);
return list.get(k-1);
}
public List<Integer> traversalTree(TreeNode root) {
List<Integer> list = new ArrayList<Integer>();

if(root != null) {
list.addAll(traversalTree(root.left));
list.add(root.val);
list.addAll(traversalTree(root.right));
}
return list;
}

}