LowestCommonAncestor.java

package LeetcodePractice;

public class LowestCommonAncestor {

    public static class TreeNode {
        int val;
        TreeNode left;
        TreeNode right;
        TreeNode(int val) { this.val = val; }
    }

    public TreeNode lowestCommonAncestor(TreeNode root, TreeNode p, TreeNode q) {
        if (root == null || root == p || root == q) return root;

        TreeNode left = lowestCommonAncestor(root.left, p, q);
        TreeNode right = lowestCommonAncestor(root.right, p, q);

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

        return left != null ? left : right;
    }

    public static void main(String[] args) {
        LowestCommonAncestor solution = new LowestCommonAncestor();

        // Construct the tree: [3,5,1,6,2,0,8,null,null,7,4]
        TreeNode root = new TreeNode(3);
        root.left = new TreeNode(5);
        root.right = new TreeNode(1);
        root.left.left = new TreeNode(6);
        root.left.right = new TreeNode(2);
        root.right.left = new TreeNode(0);
        root.right.right = new TreeNode(8);
        root.left.right.left = new TreeNode(7);
        root.left.right.right = new TreeNode(4);

        TreeNode p = root.left; // Node with value 5
        TreeNode q = root.right; // Node with value 1

        TreeNode lca = solution.lowestCommonAncestor(root, p, q);
        System.out.println("LCA of " + p.val + " and " + q.val + " is: " + lca.val);

        // Example 2: LCA of 5 and 4
        p = root.left; // 5
        q = root.left.right.right; // 4
        lca = solution.lowestCommonAncestor(root, p, q);
        System.out.println("LCA of " + p.val + " and " + q.val + " is: " + lca.val);
    }
}