Java Binary Tree Lowest Common Ancestor (LCA)

Chapter: Data Structures Last Updated: 11-11-2016 14:19:36 UTC

Program:

            /* ............... START ............... */
                
public class JavaBinaryTreeLCA {

	public static class TreeNode {
		int data;
		TreeNode left;
		TreeNode right;

		TreeNode(int data) {
			this.data = data;
		}
	}

	public static TreeNode lowestCommonAncestor(TreeNode root, TreeNode a, TreeNode b) {
		if (root == null)
			return null;
		if (root.data == a.data || root.data == b.data)
			return root;

		TreeNode left = lowestCommonAncestor(root.left, a, b);
		TreeNode right = lowestCommonAncestor(root.right, a, b);

		if (left != null && right != null)
			return root;
		if (left == null)
			return right;
		else
			return left;

	}

	public static void main(String[] args) {
		JavaBinaryTreeLCA bi = new JavaBinaryTreeLCA();
		TreeNode rootNode = createBinaryTree();
		System.out.println("Lowest common ancestor for node 5 and 30:");
		TreeNode node5 = new TreeNode(5);
		TreeNode node30 = new TreeNode(30);
		System.out.println(lowestCommonAncestor(rootNode, node5, node30).data);

	}

	public static TreeNode createBinaryTree() {

		TreeNode rootNode = new TreeNode(40);
		TreeNode node20 = new TreeNode(20);
		TreeNode node10 = new TreeNode(10);
		TreeNode node30 = new TreeNode(30);
		TreeNode node60 = new TreeNode(60);
		TreeNode node50 = new TreeNode(50);
		TreeNode node70 = new TreeNode(70);
		TreeNode node5 = new TreeNode(5);
		TreeNode node45 = new TreeNode(45);
		TreeNode node55 = new TreeNode(55);

		rootNode.left = node20;
		rootNode.right = node60;

		node20.left = node10;
		node20.right = node30;

		node60.left = node50;
		node60.right = node70;

		node10.left = node5;
		node50.right = node55;
		return rootNode;
	}
}

                /* ............... END ............... */
        

Output

Lowest common ancestor for node 5 and 30:
20

Notes:

  • Recursive Algorithm (For nodes A and B)
  • If node is null, return it
  • If we find A or B, return it.
  • Traverse left subtree and right subtree.
  • If we get both left and right for any node as not null, it will be lowest common ancestor of two given nodes.

Tags

Binary Tree Lowest Common Ancestor (LCA), Java, Data Structures

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