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Smart Contact Organiser – Closest Match in Binary Search Tree (Difficulty - Medium)


Smart Contact Organizer – Closest Match in BST

Problem Statement:

In a smart contact organiser app, names are stored in a Binary Search Tree (BST) to allow efficient alphabetical lookup.

Your task is to implement a function that finds the closest matching contact name in the BST to a given target name based on alphabetical order. If multiple names are equally close, return the one that appears first in in-order traversal.

Input:

  • tree: A list of strings representing level-order traversal of a BST. Use "null" for missing nodes.
  • target: A string representing the name you're trying to match.

Output:

Return the contact name in the BST that is alphabetically closest to the target.

Constraints:

  • Each name is a string of 1–20 alphabetic characters (uppercase/lowercase mixed)
  • The BST contains between 1 and 10⁴ nodes
  • There is at least one valid node in the input list

Example 1:

Input: [Emma, David, Sophia], Ella
Output: Emma

Explanation:
Tree structure:
       Emma
      /    \
   David  Sophia

Closest alphabetically to "Ella" is "Emma".

Example 2:

Input: [Kevin, Jake, Mike, null, null, Louis], Lucas
Output: Louis

Explanation:
        Kevin
       /     \
     Jake    Mike
             /
         Louis

"Lucas" is alphabetically closest to "Louis".

Example 3:

Input: [Zoe, Ann], Aaron
Output: Ann

Explanation:
Ann is the only node alphabetically near Aaron.

Example 4:

Input: [Mia, Emma, Olivia, null, null, Lily], Leo
Output: Lily

Explanation:
        Mia
       /   \
    Emma   Olivia
           /
         Lily

"Leo" is closest alphabetically to "Lily".


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