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Find Closest Value In BST

Understanding the problem

Given a Binary Search Tree and a target integer, we are asked to write a function that is going to return the value in the BST that is closest to the target. We can assume there is only one closest value.

For example,

      10
    /    \
   4      20
 /   \
1     6

target = 5

The closest value to the target is 4.

Iterative Approach

We are going to use a variable to keep track of the current node and initialize it to the root node of the BST. In addition, we also need a variable to keep track of the closest value in the BST so far; initialize it to the value of the root node.

While current node is not null,

• Compute the absolute difference between the value of the current node and the target. Compare the result to the absolute difference between the current closest value and the target. If we get to a smaller absolute difference, set the value of the current node as the current closest value.

• Compare the target to the current node’s value to decide which subtree we should explore:

  • If the target value is less than the current node’s value, explore the left subtree of the current node;

    • If the target value is greater than the current node’s value, explore the right subtree;

      • Otherwise break out of the while loop, since the current node’s value and the target value are equal to each other, which means there is no other value in the BST that can be closer to the target value than this value.

        • Return the closest value.

Implementation

JavaScript:

function findClosestValueInBst(tree, target) {
  let currNode = tree;
  let currClosestValue = tree.value;

  while (currNode !== null) {
    if (
      Math.abs(currNode.value - target) < Math.abs(currClosestValue - target)
    ) {
      currClosestValue = currNode.value;
    }

    if (target < currNode.value) {
      currNode = currNode.left;
    } else if (target > currNode.value) {
      currNode = currNode.right;
    } else {
      break;
    }
  }

  return currClosestValue;
}

// This is the class of the input tree. Do not edit.
class BST {
  constructor(value) {
    this.value = value;
    this.left = null;
    this.right = null;
  }
}

Complexity Analysis

• Time Complexity:

Average Case: O(log(N)), where N is the number of nodes in the Binary Search Tree.

Worst Case: O(N), when the Binary Search Tree has only one branch.

• Space Complexity: O(1).

Recursive Approach

The iterative solution above can be rewritten in the recursion form.

Implementation

JavaScript:

function findClosestValueInBst(tree, target) {
  return findClosestValueInBstImpl(tree, target, tree.value);
}

function findClosestValueInBstImpl(tree, target, closestValue) {
  if (Math.abs(tree.value - target) < Math.abs(closestValue - target)) {
    closestValue = tree.value;
  }

  if (target < tree.value && tree.left !== null) {
    return findClosestValueInBstImpl(tree.left, target, closestValue);
  }

  if (target > tree.value && tree.right !== null) {
    return findClosestValueInBstImpl(tree.right, target, closestValue);
  }

  return closestValue;
}

// This is the class of the input tree. Do not edit.
class BST {
  constructor(value) {
    this.value = value;
    this.left = null;
    this.right = null;
  }
}

Complexity Analysis

• Time Complexity:

Average Case: O(log(N)), where N is the number of nodes in the Binary Search Tree.

Worst Case: O(N), when the Binary Search Tree has only one branch.

• Space Complexity:

  Average Case: O(log(N)), where N is the number of nodes in the Binary Search Tree.

  Worst Case: O(N), when the Binary Search Tree has only one branch.

  Each recursive call to findClosestValueInBst adds a new frame on the call stack, which means we are using extra memory.In other words, we’ll be using O(h) memory, where h is the height of the tree.