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.

代码如下：

 /**
  * 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) {
         List<Integer> list=new ArrayList<>();
         list=midTree(root);

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

          if(root.left!=null)
              list.addAll(midTree(root.left));

          list.add(root.val);
          if(root.right!=null)
               list.addAll(midTree(root.right));

          return list;
     }
 }