Question.java

package Question4_6;

import CareerCupLibrary.TreeNode;

public class Question {

	static int TWO_NODES_FOUND = 2;
	static int ONE_NODE_FOUND = 1;
	static int NO_NODES_FOUND = 0;
		
	// Checks how many �special� nodes are located under this root
	public static int covers(TreeNode root, TreeNode p, TreeNode q) {
		int ret = NO_NODES_FOUND;
		if (root == null) return ret;
		if (root == p || root == q) ret += 1;
		ret += covers(root.left, p, q);
		if(ret == TWO_NODES_FOUND) // Found p and q 
			return ret;
		return ret + covers(root.right, p, q);
	}
		
	public static TreeNode commonAncestor(TreeNode root, TreeNode p, TreeNode q) {
		if (q == p && (root.left == q || root.right == q)) return root;
		int nodesFromLeft = covers(root.left, p, q); // Check left side
		if (nodesFromLeft == TWO_NODES_FOUND) {
			if(root.left == p || root.left == q) return root.left;
			else return commonAncestor(root.left, p, q);
		} else if (nodesFromLeft == ONE_NODE_FOUND) {
			if (root == p) return p;
			else if (root == q) return q;
		}

		int nodesFromRight = covers(root.right, p, q); // Check right side
		if(nodesFromRight == TWO_NODES_FOUND) {
			if(root.right == p || root.right == q) return root.right;
			else return commonAncestor(root.right, p, q);
		} else if (nodesFromRight == ONE_NODE_FOUND) {
			if (root == p) return p;
			else if (root == q) return q;
		}
		if (nodesFromLeft == ONE_NODE_FOUND && 
			nodesFromRight == ONE_NODE_FOUND) return root;
		else return null;
	}
	
	public static void main(String[] args) {
		int[] array = {1, 2, 3, 4, 5, 6, 7, 8, 9, 10};
		TreeNode root = TreeNode.createMinimalBST(array);
		TreeNode n3 = root.find(1);
		TreeNode n7 = root.find(7);
		TreeNode ancestor = commonAncestor(root, n3, n7);
		System.out.println(ancestor.data);
	}

}