230. Kth Smallest Element in a BST | 二叉排序树中查找第K小的数

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?

Credits:
Special thanks to @ts for adding this problem and creating all test cases.

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思路：根据二叉排序树的性质，我用的是中序遍历的非递归形式，访问到一个则k--，知道k==0时则当前访问的即为要找的值。

/**
 * 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) {
		int kth = 0;
		LinkedList<TreeNode> list = new LinkedList<>();
		while (root != null || !list.isEmpty()) {
			while (root != null) {
				list.push(root);
				root = root.left;
			}
			TreeNode tmp = list.poll();
			k--;
			if (k == 0) {
				kth = tmp.val;
			}
			root = tmp.right;
		}

		return kth;
	}
}