Question 4.7 has a bug. If one TreeNode is under the other one you get a NullPointerException

java/Chapter 4/Question4_7/QuestionBFixed.java

@@ -0,0 +1,48 @@
+
+import CtCILibrary.TreeNode;
+
+public class QuestionBFixed {
+  public static boolean covers(TreeNode root, TreeNode p) {
+    if (root == null) return false;
+    if (root == p) return true;
+    return covers(root.left, p) || covers(root.right, p);
+  }
+
+  public static TreeNode commonAncestorHelper(TreeNode root, TreeNode p, TreeNode q,
+      boolean covered) {
+    if (root == null) {
+      return null;
+    }
+    boolean is_p_on_left = covers(root.left, p);
+    boolean is_q_on_left = covers(root.left, q);
+    if(covered && (is_p_on_left || is_q_on_left)) {
+      return root;
+    }
+    if (is_p_on_left != is_q_on_left) { // Nodes are on different side
+      return root;
+    }
+    TreeNode child_side = is_p_on_left ? root.left : root.right;
+    return commonAncestorHelper(child_side, p, q, covered);
+  }
+
+  public static TreeNode commonAncestor(TreeNode root, TreeNode p, TreeNode q) {
+    if (!covers(root, p) || !covers(root, q)) { // Error check - one node is not in tree
+      return null;
+    }
+    boolean covered = false;
+    if(covers(p, q) || covers(p, q)) {
+      covered = true;
+    }
+    return commonAncestorHelper(root, p, q, covered);
+  }
+
+  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(6);
+    TreeNode n7 = root.find(7);
+    TreeNode ancestor = commonAncestor(root, n3, n7);
+    System.out.println(ancestor.data);
+  }
+
+}