230. Kth Smallest Element in a BST
Difficulty: Medium
Topics: Binary Search, Tree
Similar Questions:
Problem:
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.
Example 1:
Input: root = [3,1,4,null,2], k = 1 3 / \ 1 4 \ 2 Output: 1
Example 2:
Input: root = [5,3,6,2,4,null,null,1], k = 3 5 / \ 3 6 / \ 2 4 / 1 Output: 3
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?
Solutions:
/**
* Definition for a binary tree node.
* struct TreeNode {
* int val;
* TreeNode *left;
* TreeNode *right;
* TreeNode(int x) : val(x), left(NULL), right(NULL) {}
* };
*/
class Solution {
public:
int kthSmallest(TreeNode* root, int k) {
int ret = 0;
int count = 0;
helper(root, k, ret, count);
return ret;
}
bool helper(TreeNode* root, int k, int& ret, int& count) {
if (root == NULL) return false;
if (helper(root->left, k, ret, count)) return true;
++count;
if (count == k) {
ret = root->val;
return true;
}
return helper(root->right, k, ret, count);
}
};