fix: handle Null Dereference in NthUglyNumber (#5469)

Author: vil02

.inferconfig

@@ -9,7 +9,6 @@
         "src/main/java/com/thealgorithms/datastructures/trees/CreateBinaryTreeFromInorderPreorder.java",
         "src/main/java/com/thealgorithms/divideandconquer/ClosestPair.java",
         "src/main/java/com/thealgorithms/dynamicprogramming/Fibonacci.java",
-        "src/main/java/com/thealgorithms/maths/NthUglyNumber.java",
         "src/main/java/com/thealgorithms/maths/SimpsonIntegration.java",
         "src/main/java/com/thealgorithms/others/Dijkstra.java",
         "src/main/java/com/thealgorithms/sorts/TopologicalSort.java",

src/main/java/com/thealgorithms/maths/NthUglyNumber.java

@@ -2,7 +2,7 @@
 
 import java.util.ArrayList;
 import java.util.Arrays;
-import java.util.HashMap;
+import org.apache.commons.lang3.tuple.MutablePair;
 
 /**
  * @brief class computing the n-th ugly number (when they are sorted)
@@ -16,8 +16,7 @@
  */
 public class NthUglyNumber {
     private ArrayList<Long> uglyNumbers = new ArrayList<>(Arrays.asList(1L));
-    private final int[] baseNumbers;
-    private HashMap<Integer, Integer> positions = new HashMap<>();
+    private ArrayList<MutablePair<Integer, Integer>> positions = new ArrayList<>();
 
     /**
      * @brief initialized the object allowing to compute ugly numbers with given base
@@ -29,9 +28,8 @@ public class NthUglyNumber {
             throw new IllegalArgumentException("baseNumbers must be non-empty.");
         }
 
-        this.baseNumbers = baseNumbers;
         for (final var baseNumber : baseNumbers) {
-            this.positions.put(baseNumber, 0);
+            this.positions.add(MutablePair.of(baseNumber, 0));
         }
     }
 
@@ -59,21 +57,21 @@ private void addUglyNumber() {
 
     private void updatePositions() {
         final var lastUglyNumber = uglyNumbers.get(uglyNumbers.size() - 1);
-        for (final var baseNumber : baseNumbers) {
-            if (computeCandidate(baseNumber) == lastUglyNumber) {
-                positions.put(baseNumber, positions.get(baseNumber) + 1);
+        for (var entry : positions) {
+            if (computeCandidate(entry) == lastUglyNumber) {
+                entry.setValue(entry.getValue() + 1);
             }
         }
     }
 
-    private long computeCandidate(final int candidateBase) {
-        return candidateBase * uglyNumbers.get(positions.get(candidateBase));
+    private long computeCandidate(final MutablePair<Integer, Integer> entry) {
+        return entry.getKey() * uglyNumbers.get(entry.getValue());
     }
 
     private long computeMinimalCandidate() {
         long res = Long.MAX_VALUE;
-        for (final var baseNumber : baseNumbers) {
-            res = Math.min(res, computeCandidate(baseNumber));
+        for (final var entry : positions) {
+            res = Math.min(res, computeCandidate(entry));
         }
         return res;
     }