272. (Locked)Closest Binary Search Tree Value II

Difficulty: Hard

Frequency: N/A

Given a non-empty binary search tree and a target value, find k values in the BST that are closest to the target.


  • Given target value is a floating point.
  • You may assume k is always valid, that is: k ≤ total nodes.
  • You are guaranteed to have only one unique set of k values in the BST that are closest to the target.

Follow up:
Assume that the BST is balanced, could you solve it in less than O(n) runtime (where n = total nodes)?


  1. Consider implement these two helper functions:
    1. getPredecessor(N), which returns the next smaller node to N.
    2. getSuccessor(N), which returns the next larger node to N.
  2. Try to assume that each node has a parent pointer, it makes the problem much easier.
  3. Without parent pointer we just need to keep track of the path from the root to the current node using a stack.
  4. You would need two stacks to track the path in finding predecessor and successor node separately.

My solution:
Data structure:
Test cases:
Corner cases:
class Solution {
    vector<int> closestKValues(TreeNode* root, double target, int k) {
        priority_queue<pair<double, int>> pq;
        vector<int> result;
        dfs(root, pq, target, k);
        while (!pq.empty()) {
        return result;
    void dfs(TreeNode* node, priority_queue<pair<double, int>>& pq, double target, int k) {
        if (!node) return;
        pq.push(make_pair(abs(target - (double)node->val), node->val));
        if (pq.size() > k) pq.pop();
        dfs(node->left, pq, target, k);
        dfs(node->right, pq, target, k);

Another solution:
Data structure:

Things to learn:

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