230. Kth Smallest Element in a BST
Given the root of a binary search tree, and an integer k, return the kth (1-indexed) smallest element 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?
/**
* Definition for a binary tree node.
* public class TreeNode {
* int val;
* TreeNode left;
* TreeNode right;
* TreeNode() {}
* TreeNode(int val) { this.val = val; }
* TreeNode(int val, TreeNode left, TreeNode right) {
* this.val = val;
* this.left = left;
* this.right = right;
* }
* }
*/
class Solution {
public int kthSmallest(TreeNode root, int k) {
Stack<TreeNode> stack = new Stack<TreeNode>();
while(true){
while(root!=null){
stack.push(root);
root=root.left;
}
if(stack.isEmpty()){
break;
}
root = stack.pop();
if(--k==0){
return root.val;
}
root = root.right;
}
return 0;
}
}