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Problem solution · Java

Closest Nodes Queries in a Binary Search Tree

Closest Nodes Queries in a Binary Search Tree: a Java solution using binary search. Learn the idea, check the complexity, and read the full code, with credit to walkccc LeetCode Solutions.

Technique
Binary search
Source
walkccc LeetCode Solutions
Length
36 lines
Start with the idea.

Try the problem first. If you get stuck, read the approach below, then write your own solution. The full code is at the bottom.

Approach

Binary search

For Closest Nodes Queries in a Binary Search Tree, the implementation exploits a monotonic condition to discard half of the remaining search space after every check.

  1. Identify the ordered answer range or sorted search domain.
  2. Write a predicate whose truth changes only once.
  3. Move the appropriate boundary after each midpoint check and return the final feasible position.

Code notes

  • 36 lines of Java from the credited upstream file 2476.java.
  • The implementation visibly relies on sequence storage.
  • 1 loop block detected.

Complexity

Multiply the logarithmic number of midpoint checks by the cost of one predicate evaluation.

Check the problem constraints before deciding whether this complexity will pass.

Source

Code and credit

This code comes from walkccc LeetCode Solutions by P.-Y. Chen (walkccc) and is used under the MIT licence.

Open problem ↗Original source ↗
Full codeClosest Nodes Queries in a Binary Search Tree · JavaJava
Use this to learn the idea, then write your own version.
class Solution {  public List<List<Integer>> closestNodes(TreeNode root, List<Integer> queries) {    List<List<Integer>> ans = new ArrayList<>();    List<Integer> sortedVals = new ArrayList<>();     inorder(root, sortedVals);     for (final int query : queries) {      final int i = firstGreaterEqual(sortedVals, query);      // query is presented in the tree, so just use {query, query}.      if (i != sortedVals.size() && sortedVals.get(i) == query)        ans.add(Arrays.asList(query, query));      // query isn't presented in the tree, so find the cloest one if possible.      else        ans.add(Arrays.asList(i == 0 ? -1 : sortedVals.get(i - 1),                              i == sortedVals.size() ? -1 : sortedVals.get(i)));    }     return ans;  }   // Walks the BST to collect the sorted numbers.  private void inorder(TreeNode root, List<Integer> sortedVals) {    if (root == null)      return;    inorder(root.left, sortedVals);    sortedVals.add(root.val);    inorder(root.right, sortedVals);  }   private int firstGreaterEqual(List<Integer> A, int target) {    final int i = Collections.binarySearch(A, target);    return i < 0 ? -i - 1 : i;  }}

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