## Code Snippet

package com.ideserve.nilesh.questions;
/**
* <b>IDeserve <br>
* <a href="https://www.youtube.com/c/IDeserve">https://www.youtube.com/c/IDeserve</a>
* Finds Lowest Common Ancestor(LCA) of two nodes in a Binary Search Tree(BST) in O(log(n)) time.
* @author Nilesh
*/
public class LCAInBST
{
class TreeNode
{
TreeNode left;
TreeNode right;
int value;
public TreeNode(int value)
{
this.value = value;
}
}
TreeNode root;
/*
5
2 6
0 3 9
1 7
8
*/
private TreeNode createBST()
{
TreeNode n0 = new TreeNode(0);
TreeNode n1 = new TreeNode(1);
TreeNode n2 = new TreeNode(2);
TreeNode n3 = new TreeNode(3);
TreeNode n5 = new TreeNode(5);
TreeNode n6 = new TreeNode(6);
TreeNode n7 = new TreeNode(7);
TreeNode n8 = new TreeNode(8);
TreeNode n9 = new TreeNode(9);
this.root = n5;
root.left = n2;
root.right = n6;
n2.left = n0;
n2.right = n3;
n6.right = n9;
n0.right = n1;
n9.left = n7;
n7.right = n8;
return root;
}
// finds and returns node with given value in BST
private TreeNode findNode(TreeNode root, int value)
{
if (root == null)
{
return null;
}
if (value == root.value)
{
return root;
}
if (value < root.value)
{
return findNode (root.left, value);
}
else
{
return findNode (root.right, value);
}
}
// the sub-routine that actually finds LCA node for nodes n1 and n2
private TreeNode findLCA(TreeNode currentNode, TreeNode n1, TreeNode n2)
{
if (currentNode == null)
{
return null;
}
// before calling this sub-routine, we have made sure that n1.value <= n2.value
// check if currentNode is the LCA for n1 and n2
if ((n1.value < currentNode.value) && (currentNode.value < n2.value))
{
return currentNode;
}
else if ((n1.value == currentNode.value) || (currentNode.value == n2.value))
{
return currentNode;
}
// if currentNode is not the LCA for n1 and n2
TreeNode lcaNode;
if (n2.value < currentNode.value) // both n1 and n2 are in the left sub-tree of currentNode
{
lcaNode = findLCA(currentNode.left, n1, n2);
}
else // both n1 and n2 are in the right sub-tree of currentNode
{
lcaNode = findLCA(currentNode.right, n1, n2);
}
return lcaNode;
}
// this sub-routine that is entry point for clients and this in turn calls-
// findLCA(TreeNode currentNode, TreeNode n1, TreeNode n2)
public int findLCA(int value1, int value2)
{
// make sure that value1 is always less than or equal to value2
if (value2 < value1)
{
int temp = value1;
value1 = value2;
value2 = temp;
}
// find node corresponding to value1
TreeNode n1 = findNode(root, value1);
if (n1 != null)
{
// find node corresponding to value2
TreeNode n2 = findNode(root, value2);
// find LCA for these nodes
if (n2 != null)
{
TreeNode lcaNode = findLCA(root, n1, n2);
return lcaNode.value;
}
}
// if either n1 or n2 is not found
return -1;
}
public static void main(String[] args)
{
LCAInBST solution = new LCAInBST();
/*
5
2 6
0 3 9
1 7
8
*/
solution.createBST();
System.out.println(solution.findLCA(7, 8));
}
}