BAEL-7410: Finding Parent of a Node in a Binary Tree (#15736)

* adding the parent method. wrapping up structure of tree

* wrapping up recursive solution with edge cases. tests passing

* refactor parent function for clarity

* renaming Test class to suffix UnitTest

* adding iterative version tests. adding parent keeper tree and tests

* final adjustments

* adding review suggestions

* fix requested changes

* Revert "fix requested changes"

This reverts commit e52d3e6531668f99c415939c195bb4bbdd349bae.

* reverting bad commit

* adding variable for notFoundMesage

* fixing package names. fix test names

algorithms-modules/algorithms-miscellaneous-7/src/main/java/com/baeldung/algorithms/parentnodebinarytree/ParentKeeperTreeNode.java

@@ -0,0 +1,54 @@
+package com.baeldung.algorithms.parentnodebinarytree;
+
+import java.util.Objects;
+
+public class ParentKeeperTreeNode {
+
+    int value;
+    ParentKeeperTreeNode parent;
+    ParentKeeperTreeNode left;
+    ParentKeeperTreeNode right;
+
+    public ParentKeeperTreeNode(int value) {
+        this.value = value;
+    }
+
+    @Override
+    public boolean equals(Object o) {
+        if (this == o) return true;
+        if (o == null || getClass() != o.getClass()) return false;
+        ParentKeeperTreeNode treeNode = (ParentKeeperTreeNode) o;
+        return value == treeNode.value;
+    }
+
+    @Override
+    public int hashCode() {
+        return Objects.hash(value);
+    }
+
+    public void insert(int value) {
+        insert(this, value);
+    }
+
+    private void insert(ParentKeeperTreeNode currentNode, final int value) {
+        if (currentNode.left == null && value < currentNode.value) {
+            currentNode.left = new ParentKeeperTreeNode(value);
+            currentNode.left.parent = currentNode;
+            return;
+        }
+
+        if (currentNode.right == null && value > currentNode.value) {
+            currentNode.right = new ParentKeeperTreeNode(value);
+            currentNode.right.parent = currentNode;
+            return;
+        }
+
+        if (value > currentNode.value) {
+            insert(currentNode.right, value);
+        }
+
+        if (value < currentNode.value) {
+            insert(currentNode.left, value);
+        }
+    }
+}

algorithms-modules/algorithms-miscellaneous-7/src/main/java/com/baeldung/algorithms/parentnodebinarytree/TreeNode.java

@@ -0,0 +1,107 @@
+package com.baeldung.algorithms.parentnodebinarytree;
+
+import java.util.Deque;
+import java.util.LinkedList;
+import java.util.NoSuchElementException;
+import java.util.Objects;
+
+import static java.lang.String.format;
+
+public class TreeNode {
+    int value;
+    TreeNode left;
+    TreeNode right;
+
+    public TreeNode(int value) {
+        this.value = value;
+    }
+
+    @Override
+    public boolean equals(Object o) {
+        if (this == o) return true;
+        if (o == null || getClass() != o.getClass()) return false;
+        TreeNode treeNode = (TreeNode) o;
+        return value == treeNode.value;
+    }
+
+    @Override
+    public int hashCode() {
+        return Objects.hash(value);
+    }
+
+    public void insert(int value) {
+        insert(this, value);
+    }
+
+    private void insert(TreeNode currentNode, final int value) {
+        if (currentNode.left == null && value < currentNode.value) {
+            currentNode.left = new TreeNode(value);
+            return;
+        }
+
+        if (currentNode.right == null && value > currentNode.value) {
+            currentNode.right = new TreeNode(value);
+            return;
+        }
+
+        if (value > currentNode.value) {
+            insert(currentNode.right, value);
+        }
+
+        if (value < currentNode.value) {
+            insert(currentNode.left, value);
+        }
+    }
+
+    public TreeNode parent(int target) throws NoSuchElementException {
+        return parent(this, new TreeNode(target));
+    }
+
+    private TreeNode parent(TreeNode current, TreeNode target) throws NoSuchElementException {
+        if (target.equals(current) || current == null) {
+            throw new NoSuchElementException(format("No parent node found for 'target.value=%s' " +
+                            "The target is not in the tree or the target is the topmost root node.",
+                    target.value));
+        }
+
+        if (target.equals(current.left) || target.equals(current.right)) {
+            return current;
+        }
+
+        return parent(target.value < current.value ? current.left : current.right, target);
+    }
+
+    public TreeNode iterativeParent(int target) {
+        return iterativeParent(this, new TreeNode(target));
+    }
+
+    private TreeNode iterativeParent(TreeNode current, TreeNode target) {
+        Deque<TreeNode> parentCandidates = new LinkedList<>();
+
+        String notFoundMessage = format("No parent node found for 'target.value=%s' " +
+                        "The target is not in the tree or the target is the topmost root node.",
+                target.value);
+
+        if (target.equals(current)) {
+            throw new NoSuchElementException(notFoundMessage);
+        }
+
+        while (current != null || !parentCandidates.isEmpty()) {
+
+            while (current != null) {
+                parentCandidates.addFirst(current);
+                current = current.left;
+            }
+
+            current = parentCandidates.pollFirst();
+
+            if (target.equals(current.left) || target.equals(current.right)) {
+                return current;
+            }
+
+            current = current.right;
+        }
+
+        throw new NoSuchElementException(notFoundMessage);
+    }
+}

algorithms-modules/algorithms-miscellaneous-7/src/test/java/com/baeldung/algorithms/parentnodebinarytree/BinaryTreeParentNodeFinderUnitTest.java

@@ -0,0 +1,83 @@
+package com.baeldung.algorithms.parentnodebinarytree;
+
+import org.junit.jupiter.api.BeforeEach;
+import org.junit.jupiter.api.Test;
+
+import java.util.NoSuchElementException;
+
+import static org.junit.jupiter.api.Assertions.*;
+
+class BinaryTreeParentNodeFinderUnitTest {
+
+    private TreeNode subject;
+
+    @BeforeEach
+    void setUp() {
+        subject = new TreeNode(8);
+        subject.insert(5);
+        subject.insert(12);
+        subject.insert(3);
+        subject.insert(7);
+        subject.insert(1);
+        subject.insert(4);
+        subject.insert(11);
+        subject.insert(14);
+        subject.insert(13);
+        subject.insert(16);
+    }
+
+    @Test
+    void givenBinaryTree_whenFindParentNode_thenReturnCorrectParentNode() {
+        assertEquals(8, subject.parent(5).value);
+        assertEquals(5, subject.parent(3).value);
+        assertEquals(5, subject.parent(7).value);
+        assertEquals(3, subject.parent(4).value);
+        assertEquals(3, subject.parent(1).value);
+        assertEquals(8, subject.parent(12).value);
+        assertEquals(12, subject.parent(14).value);
+        assertEquals(12, subject.parent(11).value);
+        assertEquals(14, subject.parent(16).value);
+        assertEquals(14, subject.parent(13).value);
+        assertThrows(NoSuchElementException.class, () -> subject.parent(1231));
+        assertThrows(NoSuchElementException.class, () -> subject.parent(8));
+    }
+
+    @Test
+    void givenBinaryTree_whenFindParentNodeIteratively_thenReturnCorrectParentNode() {
+        assertEquals(8, subject.iterativeParent(5).value);
+        assertEquals(5, subject.iterativeParent(3).value);
+        assertEquals(5, subject.iterativeParent(7).value);
+        assertEquals(3, subject.iterativeParent(4).value);
+        assertEquals(3, subject.iterativeParent(1).value);
+        assertEquals(8, subject.iterativeParent(12).value);
+        assertEquals(12, subject.iterativeParent(14).value);
+        assertEquals(12, subject.iterativeParent(11).value);
+        assertEquals(14, subject.iterativeParent(16).value);
+        assertEquals(14, subject.iterativeParent(13).value);
+        assertThrows(NoSuchElementException.class, () -> subject.iterativeParent(1231));
+        assertThrows(NoSuchElementException.class, () -> subject.iterativeParent(8));
+    }
+
+    @Test
+    void givenParentKeeperBinaryTree_whenGetParent_thenReturnCorrectParent() {
+        ParentKeeperTreeNode subject = new ParentKeeperTreeNode(8);
+        subject.insert(5);
+        subject.insert(12);
+        subject.insert(3);
+        subject.insert(7);
+        subject.insert(1);
+        subject.insert(4);
+        subject.insert(11);
+        subject.insert(14);
+        subject.insert(13);
+        subject.insert(16);
+
+        assertNull(subject.parent);
+        assertEquals(8, subject.left.parent.value);
+        assertEquals(8, subject.right.parent.value);
+        assertEquals(5, subject.left.left.parent.value);
+        assertEquals(5, subject.left.right.parent.value);
+
+        // tests for other nodes
+    }
+}