230-kth-smallest-element-in-a-bst

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Description

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

 

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

 

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?

Solution(Python)

class Solution:
    def kthSmallest(self, root, k):
        """
        :type root: TreeNode
        :type k: int
        :rtype: int
        """

        cur = root

        while cur:
            if cur.left is None:
                k -= 1
                if k == 0:
                    return cur.val
                cur = cur.right
            else:
                pre = cur.left

                while pre.right and pre.right is not cur:
                    pre = pre.right

                if pre.right is None:
                    pre.right = cur
                    cur = cur.left
                else:
                    pre.right = None
                    k -= 1
                    if k == 0:
                        return cur.val
                    cur = cur.right