230. Kth Smallest Element in a BST

题目：

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?

Hint:

1. Try to utilize the property of a BST.
2. What if you could modify the BST node's structure?
3. The optimal runtime complexity is O(height of BST).

链接：  http://leetcode.com/problems/kth-smallest-element-in-a-bst/

7/25/2017

2ms, 20%

好几天终于有道自己做的题了。。。iterative inorder traversal

public class Solution {
    public int kthSmallest(TreeNode root, int k) {
        Stack<TreeNode> stack = new Stack<>();
        TreeNode node = root;
        int kthSml = 0;
        
        while((node != null || !stack.isEmpty()) && k > 0) {
            while (node != null) {
                stack.push(node);
                node = node.left;
            }
            node = stack.pop();
            kthSml = node.val;
            k--;
            node = node.right;
        }
        return kthSml;
    }
}

别人的答案

可以利用DFS求root的count，然后再从左右两边找

https://discuss.leetcode.com/topic/17810/3-ways-implemented-in-java-python-binary-search-in-order-iterative-recursive

python generator

https://discuss.leetcode.com/topic/18279/pythonic-approach-with-generator

更多讨论

https://discuss.leetcode.com/category/238/kth-smallest-element-in-a-bst