Fix checkstyle warnings and refactor tree classes

spotbugs-exclude.xml

@@ -2,7 +2,7 @@
 <FindBugsFilter>
     <Match>
         <!-- The Field is used to retain reified type info on a field and is not UNREAD -->
-        <Class name="org.nintynine.problems.BTreeP56$Node"/>
+        <Class name="org.nintynine.problems.BtreeP56$Node"/>
         <Bug pattern="URF_UNREAD_FIELD"/>
     </Match>
 </FindBugsFilter>

src/main/java/org/nintynine/problems/BTree54.java → src/main/java/org/nintynine/problems/Btree54.java

@@ -3,12 +3,12 @@
 import java.util.Objects;
 
 /** P54A: Check whether a given expression represents a binary tree */
-public class BTree54 {
+public class Btree54 {
   /** Represents a node in the binary tree expression */
-  public static class BTree54Node {
+  public static class Btree54Node {
     private final String value;
-    private final BTree54Node left;
-    private final BTree54Node right;
+    private final Btree54Node left;
+    private final Btree54Node right;
 
     /**
      * Constructs a binary tree node
@@ -17,7 +17,7 @@ public static class BTree54Node {
      * @param left Left child node or null
      * @param right Right child node or null
      */
-    public BTree54Node(String value, BTree54Node left, BTree54Node right) {
+    public Btree54Node(String value, Btree54Node left, Btree54Node right) {
       this.value = Objects.requireNonNull(value, "Node value cannot be null");
       this.left = left;
       this.right = right;
@@ -28,14 +28,14 @@ public BTree54Node(String value, BTree54Node left, BTree54Node right) {
      *
      * @param value The value at this node
      */
-    public BTree54Node(String value) {
+    public Btree54Node(String value) {
       this(value, null, null);
     }
 
     @Override
     public boolean equals(Object o) {
       if (this == o) return true;
-      if (!(o instanceof BTree54Node bTree54Node)) return false;
+      if (!(o instanceof Btree54Node bTree54Node)) return false;
       return Objects.equals(value, bTree54Node.value)
           && Objects.equals(left, bTree54Node.left)
           && Objects.equals(right, bTree54Node.right);
@@ -60,7 +60,7 @@ public String toString() {
     }
   }
 
-  private BTree54() {} // Prevent instantiation
+  private Btree54() {} // Prevent instantiation
 
   private static boolean isValidValue(String value) {
     // Check for any special characters or improper formatting
@@ -113,7 +113,7 @@ public static boolean isTree(String expression) {
    * @return The root node of the parsed tree
    * @throws IllegalArgumentException if the expression is invalid
    */
-  public static BTree54Node parseTree(String expression) {
+  public static Btree54Node parseTree(String expression) {
     if (expression == null) {
       throw new IllegalArgumentException("Expression cannot be null");
     }
@@ -123,7 +123,7 @@ public static BTree54Node parseTree(String expression) {
     // Handle single value case
     if (!expression.startsWith("(")) {
       if (isValidValue(expression)) {
-        return new BTree54Node(expression);
+        return new Btree54Node(expression);
       }
       throw new IllegalArgumentException("Invalid value: " + expression);
     }
@@ -148,10 +148,10 @@ public static BTree54Node parseTree(String expression) {
       throw new IllegalArgumentException("Invalid node value");
     }
 
-    BTree54Node left = "nil".equals(leftExpr) ? null : parseTree(leftExpr);
-    BTree54Node right = "nil".equals(rightExpr) ? null : parseTree(rightExpr);
+    Btree54Node left = "nil".equals(leftExpr) ? null : parseTree(leftExpr);
+    Btree54Node right = "nil".equals(rightExpr) ? null : parseTree(rightExpr);
 
-    return new BTree54Node(value, left, right);
+    return new Btree54Node(value, left, right);
   }
 
   @SuppressWarnings("java:S5852")

src/main/java/org/nintynine/problems/BTreeP56.java → src/main/java/org/nintynine/problems/BtreeP56.java

@@ -1,6 +1,11 @@
 package org.nintynine.problems;
 
-public class BTreeP56<T> {
+/**
+ * Simple binary tree used for symmetry checks in problem 56.
+ *
+ * @param <T> node value type
+ */
+public class BtreeP56<T> {
   private Node<T> root;
 
   private static class Node<T> {
@@ -15,7 +20,7 @@ private static class Node<T> {
     }
   }
 
-  public BTreeP56() {
+  public BtreeP56() {
     this.root = null;
   }
 
@@ -24,27 +29,37 @@ public void setRoot(T value) {
   }
 
   public void addLeft(T value) {
-    if (root == null) throw new IllegalStateException("Tree has no root");
+    if (root == null) {
+      throw new IllegalStateException("Tree has no root");
+    }
     root.left = new Node<>(value);
   }
 
   public void addRight(T value) {
-    if (root == null) throw new IllegalStateException("Tree has no root");
+    if (root == null) {
+      throw new IllegalStateException("Tree has no root");
+    }
     root.right = new Node<>(value);
   }
 
   public boolean isSymmetric() {
-    if (root == null) return true;
+    if (root == null) {
+      return true;
+    }
     return isMirror(root.left, root.right);
   }
 
   @SuppressWarnings("java:S2234") // or just "S2234"
   private boolean isMirror(Node<T> left, Node<T> right) {
     // If both nodes are null, they are mirror images
-    if (left == null && right == null) return true;
+    if (left == null && right == null) {
+      return true;
+    }
 
     // If only one node is null, they are not mirror images
-    if (left == null || right == null) return false;
+    if (left == null || right == null) {
+      return false;
+    }
 
     // Check if the structure is mirrored
     return isMirror(left.left, right.right) && isMirror(left.right, right.left);

src/main/java/org/nintynine/problems/BTreeP57.java → src/main/java/org/nintynine/problems/BtreeP57.java

@@ -1,6 +1,12 @@
 package org.nintynine.problems;
 
-public class BTreeP57<T extends Comparable<T>> {
+/**
+ * Binary tree utilities for problem 57.
+ *
+ * @param <T> type of node values
+ */
+@SuppressWarnings("checkstyle:AbbreviationAsWordInName")
+public class BtreeP57<T extends Comparable<T>> {
   private Node<T> root;
 
   private static class Node<T> {
@@ -15,10 +21,15 @@ private static class Node<T> {
     }
   }
 
-  public BTreeP57() {
+  public BtreeP57() {
     this.root = null;
   }
 
+  /**
+   * Constructs the tree by inserting all given values.
+   *
+   * @param values values to insert
+   */
   public void construct(T[] values) {
     for (T value : values) {
       insert(value);
@@ -44,6 +55,11 @@ private Node<T> insertRec(Node<T> node, T value) {
     return node;
   }
 
+  /**
+   * Checks if this tree is symmetric.
+   *
+   * @return {@code true} if the tree is symmetric, otherwise {@code false}
+   */
   public boolean isSymmetric() {
     return root == null || isMirror(root.left, root.right);
   }