230. Kth Smallest Element in a BST

Medium (Trung bình) C++ 🔗 Xem trên LeetCode

📋 Đề Bài

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?

🧠 Thuật Toán & Kỹ Thuật

Tree Traversal (Duyệt cây)
⏱️ Thời gian O(n)
💾 Không gian O(1)

💻 Lời Giải

C++ 0230-kth-smallest-element-in-a-bst.cpp 
/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode() : val(0), left(nullptr), right(nullptr) {}
 *     TreeNode(int x) : val(x), left(nullptr), right(nullptr) {}
 *     TreeNode(int x, TreeNode *left, TreeNode *right) : val(x), left(left), right(right) {}
 * };
 */

class Solution {
private:
    int k;
    int ans;
    
public:

    void solve(TreeNode *node) {
        if(!node) return;
        solve(node->left);
        k--;
        if (k == 0) {
            ans = node->val;
        }
        solve(node->right);
    }
    int kthSmallest(TreeNode* root, int k) {
        this->k = k;
        solve(root);
        return ans;
    }
};