diff --git a/src/data-structures/binary-tree/avl-tree.ts b/src/data-structures/binary-tree/avl-tree.ts
index 9413f47..caafbd1 100644
--- a/src/data-structures/binary-tree/avl-tree.ts
+++ b/src/data-structures/binary-tree/avl-tree.ts
@@ -14,7 +14,7 @@ import type {
   BSTNodeKeyOrNode,
   BTNodeExemplar
 } from '../../types';
-import { BTNCallback } from '../../types';
+import { BTNCallback, BTNodeKeyOrNode } from '../../types';
 import { IBinaryTree } from '../../interfaces';
 
 export class AVLTreeNode<K = any, V = any, N extends AVLTreeNode<K, V, N> = AVLTreeNodeNested<K, V>> extends BSTNode<K, V, N> {
@@ -90,6 +90,16 @@ export class AVLTree<K = any, V = any, N extends AVLTreeNode<K, V, N> = AVLTreeN
     return exemplar instanceof AVLTreeNode;
   }
 
+  /**
+   * The function "isNotNodeInstance" checks if a potential key is a K.
+   * @param {any} potentialKey - The potentialKey parameter is of type any, which means it can be any
+   * data type.
+   * @returns a boolean value indicating whether the potentialKey is of type number or not.
+   */
+  override isNotNodeInstance(potentialKey: BTNodeKeyOrNode<K, N>): potentialKey is K {
+    return !(potentialKey instanceof AVLTreeNode)
+  }
+
   /**
    * Time Complexity: O(log n) - logarithmic time, where "n" is the number of nodes in the tree. The add method of the superclass (BST) has logarithmic time complexity.
    * Space Complexity: O(1) - constant space, as it doesn't use additional data structures that scale with input size.
diff --git a/src/data-structures/binary-tree/binary-tree.ts b/src/data-structures/binary-tree/binary-tree.ts
index 00a0f93..421da3c 100644
--- a/src/data-structures/binary-tree/binary-tree.ts
+++ b/src/data-structures/binary-tree/binary-tree.ts
@@ -1236,8 +1236,8 @@ export class BinaryTree<K = any, V = any, N extends BinaryTreeNode<K, V, N> = Bi
    * @param {any} node - The parameter `node` is of type `any`, which means it can be any data type.
    * @returns a boolean value.
    */
-  isRealNode(node: any): node is N {
-    return node instanceof BinaryTreeNode && node.key.toString() !== 'NaN';
+  isRealNode(node: BTNodeExemplar<K, V, N>): node is N {
+    return node instanceof BinaryTreeNode && String(node.key) !== 'NaN';
   }
 
   /**
@@ -1245,8 +1245,8 @@ export class BinaryTree<K = any, V = any, N extends BinaryTreeNode<K, V, N> = Bi
    * @param {any} node - The parameter `node` is of type `any`, which means it can be any data type.
    * @returns a boolean value.
    */
-  isNIL(node: any) {
-    return node instanceof BinaryTreeNode && node.key.toString() === 'NaN';
+  isNIL(node: BTNodeExemplar<K, V, N>) {
+    return node instanceof BinaryTreeNode && String(node.key) === 'NaN';
   }
 
   /**
@@ -1254,12 +1254,12 @@ export class BinaryTree<K = any, V = any, N extends BinaryTreeNode<K, V, N> = Bi
    * @param {any} node - The parameter `node` is of type `any`, which means it can be any data type.
    * @returns a boolean value.
    */
-  isNodeOrNull(node: any): node is N | null {
+  isNodeOrNull(node: BTNodeExemplar<K, V, N>): node is N | null {
     return this.isRealNode(node) || node === null;
   }
 
   /**
-   * The function "isNotNodeInstance" checks if a potential key is a number.
+   * The function "isNotNodeInstance" checks if a potential key is a K.
    * @param {any} potentialKey - The potentialKey parameter is of type any, which means it can be any
    * data type.
    * @returns a boolean value indicating whether the potentialKey is of type number or not.
@@ -1606,26 +1606,24 @@ export class BinaryTree<K = any, V = any, N extends BinaryTreeNode<K, V, N> = Bi
   }
 
   /**
-   * Time complexity: O(n)
-   * Space complexity: O(n)
+   * Time Complexity: O(log n)
+   * Space Complexity: O(1)
    */
 
-  getPredecessor(node: N): N;
-
   /**
-   * The function `getPredecessor` returns the predecessor node of a given node in a binary tree.
-   * @param {K | N | null | undefined} node - The `node` parameter can be of type `K`, `N`,
-   * `null`, or `undefined`.
-   * @returns The function `getPredecessor` returns a value of type `N | undefined`.
+   * Time Complexity: O(log n)
+   * Space Complexity: O(1)
+   *
+   * The function returns the predecessor of a given node in a tree.
+   * @param {N} node - The parameter `node` is of type `RedBlackTreeNode`, which represents a node in a
+   * tree.
+   * @returns the predecessor of the given 'node'.
    */
-  getPredecessor(node: BTNodeKeyOrNode<K, N>): N | undefined {
-    node = this.ensureNode(node);
-    if (!this.isRealNode(node)) return undefined;
-
-    if (node.left) {
+  getPredecessor(node: N): N {
+    if (this.isRealNode(node.left)) {
       let predecessor: N | null | undefined = node.left;
       while (!this.isRealNode(predecessor) || (this.isRealNode(predecessor.right) && predecessor.right !== node)) {
-        if (predecessor) {
+        if (this.isRealNode(predecessor)) {
           predecessor = predecessor.right;
         }
       }
@@ -1642,15 +1640,16 @@ export class BinaryTree<K = any, V = any, N extends BinaryTreeNode<K, V, N> = Bi
    * after the given node in the inorder traversal of the binary tree.
    */
   getSuccessor(x?: K | N | null): N | null | undefined {
-    x = this.ensureNode(x);
-    if (!x) return undefined;
 
-    if (x.right) {
+    x = this.ensureNode(x);
+    if (!this.isRealNode(x)) return undefined;
+
+    if (this.isRealNode(x.right)) {
       return this.getLeftMost(x.right);
     }
 
     let y: N | null | undefined = x.parent;
-    while (y && y && x === y.right) {
+    while (this.isRealNode(y) && x === y.right) {
       x = y;
       y = y.parent;
     }
diff --git a/src/data-structures/binary-tree/bst.ts b/src/data-structures/binary-tree/bst.ts
index 6d2ebd2..1347e0c 100644
--- a/src/data-structures/binary-tree/bst.ts
+++ b/src/data-structures/binary-tree/bst.ts
@@ -14,7 +14,7 @@ import type {
   BTNodeExemplar,
   BTNodePureExemplar
 } from '../../types';
-import { BSTVariant, CP, IterationType } from '../../types';
+import { BSTVariant, BTNodeKeyOrNode, CP, IterationType } from '../../types';
 import { BinaryTree, BinaryTreeNode } from './binary-tree';
 import { IBinaryTree } from '../../interfaces';
 import { Queue } from '../queue';
@@ -442,6 +442,16 @@ export class BST<K = any, V = any, N extends BSTNode<K, V, N> = BSTNode<K, V, BS
     }
   }
 
+  /**
+   * The function "isNotNodeInstance" checks if a potential key is a K.
+   * @param {any} potentialKey - The potentialKey parameter is of type any, which means it can be any
+   * data type.
+   * @returns a boolean value indicating whether the potentialKey is of type number or not.
+   */
+  override isNotNodeInstance(potentialKey: BTNodeKeyOrNode<K, N>): potentialKey is K {
+    return !(potentialKey instanceof BSTNode)
+  }
+
   /**
    * Time Complexity: O(log n) - Average case for a balanced tree.
    * Space Complexity: O(log n) - Space for the recursive call stack in the worst case.
diff --git a/src/data-structures/binary-tree/rb-tree.ts b/src/data-structures/binary-tree/rb-tree.ts
index fec27e7..cad7928 100644
--- a/src/data-structures/binary-tree/rb-tree.ts
+++ b/src/data-structures/binary-tree/rb-tree.ts
@@ -11,6 +11,7 @@ import {
   BSTNodeKeyOrNode,
   BTNCallback,
   BTNodeExemplar,
+  BTNodeKeyOrNode,
   IterationType,
   RBTNColor,
   RBTreeOptions,
@@ -115,6 +116,16 @@ export class RedBlackTree<K = any, V = any, N extends RedBlackTreeNode<K, V, N>
     return exemplar instanceof RedBlackTreeNode;
   }
 
+  /**
+   * The function "isNotNodeInstance" checks if a potential key is a K.
+   * @param {any} potentialKey - The potentialKey parameter is of type any, which means it can be any
+   * data type.
+   * @returns a boolean value indicating whether the potentialKey is of type number or not.
+   */
+  override isNotNodeInstance(potentialKey: BTNodeKeyOrNode<K, N>): potentialKey is K {
+    return !(potentialKey instanceof RedBlackTreeNode)
+  }
+
   /**
    * The function `exemplarToNode` takes an exemplar and converts it into a node object if possible.
    * @param exemplar - The `exemplar` parameter is of type `BTNodeExemplar<K, V, N>`, where:
@@ -300,7 +311,8 @@ export class RedBlackTree<K = any, V = any, N extends RedBlackTreeNode<K, V, N>
    */
 
   override isRealNode(node: N | undefined): node is N {
-    return node !== this.Sentinel && node !== undefined;
+    if (node === this.Sentinel || node === undefined) return false;
+    return node instanceof RedBlackTreeNode;
   }
 
   getNode<C extends BTNCallback<N, K>>(
@@ -362,38 +374,12 @@ export class RedBlackTree<K = any, V = any, N extends RedBlackTreeNode<K, V, N>
   }
 
   /**
-   * Time Complexity: O(log n) on average (where n is the number of nodes in the tree)
+   * Time Complexity: O(log n)
    * Space Complexity: O(1)
    */
 
   /**
-   * Time Complexity: O(log n) on average (where n is the number of nodes in the tree)
-   * Space Complexity: O(1)
-   *
-   * The function returns the successor of a given node in a red-black tree.
-   * @param {RedBlackTreeNode} x - RedBlackTreeNode - The node for which we want to find the successor.
-   * @returns the successor of the given RedBlackTreeNode.
-   */
-  override getSuccessor(x: N): N | undefined {
-    if (x.right !== this.Sentinel) {
-      return this.getLeftMost(x.right) ?? undefined;
-    }
-
-    let y: N | undefined = x.parent;
-    while (y !== this.Sentinel && y !== undefined && x === y.right) {
-      x = y;
-      y = y.parent;
-    }
-    return y;
-  }
-
-  /**
-   * Time Complexity: O(log n) on average (where n is the number of nodes in the tree)
-   * Space Complexity: O(1)
-   */
-
-  /**
-   * Time Complexity: O(log n) on average (where n is the number of nodes in the tree)
+   * Time Complexity: O(log n)
    * Space Complexity: O(1)
    *
    * The function returns the predecessor of a given node in a red-black tree.
@@ -402,12 +388,12 @@ export class RedBlackTree<K = any, V = any, N extends RedBlackTreeNode<K, V, N>
    * @returns the predecessor of the given RedBlackTreeNode 'x'.
    */
   override getPredecessor(x: N): N {
-    if (x.left !== this.Sentinel) {
-      return this.getRightMost(x.left!)!;
+    if (this.isRealNode(x.left)) {
+      return this.getRightMost(x.left)!;
     }
 
     let y: N | undefined = x.parent;
-    while (y !== this.Sentinel && x === y!.left) {
+    while (this.isRealNode(y) && x === y.left) {
       x = y!;
       y = y!.parent;
     }
diff --git a/src/data-structures/binary-tree/tree-multimap.ts b/src/data-structures/binary-tree/tree-multimap.ts
index 79fb5a8..70fd770 100644
--- a/src/data-structures/binary-tree/tree-multimap.ts
+++ b/src/data-structures/binary-tree/tree-multimap.ts
@@ -6,7 +6,14 @@
  * @license MIT License
  */
 import type { BSTNodeKeyOrNode, BTNodeExemplar, TreeMultimapNodeNested, TreeMultimapOptions } from '../../types';
-import { BiTreeDeleteResult, BTNCallback, FamilyPosition, IterationType, TreeMultimapNested } from '../../types';
+import {
+  BiTreeDeleteResult,
+  BTNCallback,
+  BTNodeKeyOrNode,
+  FamilyPosition,
+  IterationType,
+  TreeMultimapNested
+} from '../../types';
 import { IBinaryTree } from '../../interfaces';
 import { AVLTree, AVLTreeNode } from './avl-tree';
 
@@ -85,6 +92,15 @@ export class TreeMultimap<K = any, V = any, N extends TreeMultimapNode<K, V, N>
     return exemplar instanceof TreeMultimapNode;
   }
 
+  /**
+   * The function "isNotNodeInstance" checks if a potential key is a K.
+   * @param {any} potentialKey - The potentialKey parameter is of type any, which means it can be any
+   * data type.
+   * @returns a boolean value indicating whether the potentialKey is of type number or not.
+   */
+  override isNotNodeInstance(potentialKey: BTNodeKeyOrNode<K, N>): potentialKey is K {
+    return !(potentialKey instanceof TreeMultimapNode)
+  }
 
   /**
    * The function `exemplarToNode` converts an exemplar object into a node object.