【leetcode】Kth Smallest Element in a BST

一、问题描述

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.

二、问题分析

结合BST的特点，很容易想到的是BST的先序遍历正好是一个有序递增序列，kth即为该序列的第k个直接get到即可。

三、Java AC代码

public int kthSmallest(TreeNode root, int k) {
		List<Integer> list = new ArrayList<Integer>();
		preOrderTraverse(root, list);
		return list.get(k-1);
	}
	public void preOrderTraverse(TreeNode root, List<Integer> list){
		if (root == null) {
			return ;
		}
		preOrderTraverse(root.left, list);
		list.add(root.val);
		preOrderTraverse(root.right, list);
	}