230. Kth Smallest Element in a BST

1. Question

Given the root of a binary search tree, and an integer k, return the k^th smallest value (1-indexed) of all the values of the nodes in the tree.

2. Examples

Example 1:

Input: root = [3,1,4,null,2], k = 1
Output: 1

Example 2:

Input: root = [5,3,6,2,4,null,null,1], k = 3
Output: 3

3. Constraints

• The number of nodes in the tree is n.
• 1 <= k <= n <= 10⁴
• 0 <= Node.val <= 10⁴

Follow up: If the BST is modified often (i.e., we can do insert and delete operations) and you need to find the kth smallest frequently, how would you optimize?

4. References

来源：力扣（LeetCode） 链接：https://leetcode-cn.com/problems/kth-smallest-element-in-a-bst 著作权归领扣网络所有。商业转载请联系官方授权，非商业转载请注明出处。

5. Solutions

class Solution {
  public int kthSmallest(TreeNode root, int k) {
    Stack<TreeNode> stack = new Stack<>();
    while (root != null || !stack.isEmpty()) {
      while (root != null) {
        stack.push(root);
        root = root.left;
      }
      root = stack.pop();
      if (--k == 0) {
        return root.val;
      }
      root = root.right;
    }
    return -1;
  }
}