Merge branch 'master' into refactor/PostfixToInfix

Author: alxkm

src/main/java/com/thealgorithms/stacks/NextGreaterElement.java

@@ -1,53 +1,34 @@
 package com.thealgorithms.stacks;
 
-import java.util.Arrays;
 import java.util.Stack;
 
-/*
-    Given an array "input" you need to print the first greater element for each element.
-    For a given element x of an array, the Next greater element of that element is the
-    first greater element to the right side of it. If no such element is present print -1.
-
-    Example
-    input = { 2, 7, 3, 5, 4, 6, 8 };
-    At i = 0
-    Next greater element between (1 to n) is 7
-    At i = 1
-    Next greater element between (2 to n) is 8
-    At i = 2
-    Next greater element between (3 to n) is 5
-    At i = 3
-    Next greater element between (4 to n) is 6
-    At i = 4
-    Next greater element between (5 to n) is 6
-    At i = 5
-    Next greater element between (6 to n) is 8
-    At i = 6
-    Next greater element between (6 to n) is -1
-
-    result : [7, 8, 5, 6, 6, 8, -1]
-
-    1. If the stack is empty Push an element in the stack.
-    2. If the stack is not empty:
-        a.  compare the top element of the stack with next.
-        b.  If next is greater than the top element, Pop element from the stack.
-            next is the next greater element for the popped element.
-        c.  Keep popping from the stack while the popped element is smaller
-            than next. next becomes the next greater element for all such
-            popped elements.
-        d. Finally, push the next in the stack.
-
-    3. If elements are left in stack after completing while loop then their Next greater element is
-   -1.
+/**
+ * Utility class to find the next greater element for each element in a given integer array.
+ *
+ * <p>The next greater element for an element x is the first greater element on the right side of x in the array.
+ * If no such element exists, the result will contain 0 for that position.</p>
+ *
+ * <p>Example:</p>
+ * <pre>
+ * Input:  {2, 7, 3, 5, 4, 6, 8}
+ * Output: {7, 0, 5, 6, 6, 8, 0}
+ * </pre>
  */
-
 public final class NextGreaterElement {
     private NextGreaterElement() {
     }
 
+    /**
+     * Finds the next greater element for each element in the given array.
+     *
+     * @param array the input array of integers
+     * @return an array where each element is replaced by the next greater element on the right side in the input array,
+     * or 0 if there is no greater element.
+     * @throws IllegalArgumentException if the input array is null
+     */
     public static int[] findNextGreaterElements(int[] array) {
         if (array == null) {
-            return array;
+            throw new IllegalArgumentException("Input array cannot be null");
         }
 
         int[] result = new int[array.length];
@@ -62,10 +43,4 @@ public static int[] findNextGreaterElements(int[] array) {
 
         return result;
     }
-
-    public static void main(String[] args) {
-        int[] input = {2, 7, 3, 5, 4, 6, 8};
-        int[] result = findNextGreaterElements(input);
-        System.out.println(Arrays.toString(result));
-    }
 }

src/main/java/com/thealgorithms/stacks/NextSmallerElement.java

@@ -3,70 +3,57 @@
 import java.util.Arrays;
 import java.util.Stack;
 
-/*
-    Given an array "input" you need to print the first smaller element for each element to the left
-   side of an array. For a given element x of an array, the Next Smaller element of that element is
-   the first smaller element to the left side of it. If no such element is present print -1.
-
-    Example
-    input = { 2, 7, 3, 5, 4, 6, 8 };
-    At i = 0
-    No elements to left of it : -1
-    At i = 1
-    Next smaller element between (0 , 0) is 2
-    At i = 2
-    Next smaller element between (0 , 1) is 2
-    At i = 3
-    Next smaller element between (0 , 2) is 3
-    At i = 4
-    Next smaller element between (0 , 3) is 3
-    At i = 5
-    Next smaller element between (0 , 4) is 4
-    At i = 6
-    Next smaller element between (0 , 5) is 6
-
-    result : [-1, 2, 2, 3, 3, 4, 6]
-
-    1) Create a new empty stack st
-
-    2) Iterate over array "input" , where "i"  goes from 0 to input.length -1.
-        a) We are looking for value just smaller than `input[i]`. So keep popping from "stack"
-           till elements in "stack.peek() >= input[i]" or stack becomes empty.
-        b) If the stack is non-empty, then the top element is our previous element. Else the
-   previous element does not exist. c) push input[i] in stack. 3) If elements are left then their
-   answer is -1
+/**
+ * Utility class to find the next smaller element for each element in a given integer array.
+ *
+ * <p>The next smaller element for an element x is the first smaller element on the left side of x in the array.
+ * If no such element exists, the result will contain -1 for that position.</p>
+ *
+ * <p>Example:</p>
+ * <pre>
+ * Input:  {2, 7, 3, 5, 4, 6, 8}
+ * Output: [-1, 2, 2, 3, 3, 4, 6]
+ * </pre>
  */
-
 public final class NextSmallerElement {
     private NextSmallerElement() {
     }
 
+    /**
+     * Finds the next smaller element for each element in the given array.
+     *
+     * @param array the input array of integers
+     * @return an array where each element is replaced by the next smaller element on the left side in the input array,
+     * or -1 if there is no smaller element.
+     * @throws IllegalArgumentException if the input array is null
+     */
     public static int[] findNextSmallerElements(int[] array) {
-        // base case
         if (array == null) {
-            return array;
+            throw new IllegalArgumentException("Input array cannot be null");
         }
-        Stack<Integer> stack = new Stack<>();
+
         int[] result = new int[array.length];
+        Stack<Integer> stack = new Stack<>();
+
+        // Initialize all elements to -1 (in case there is no smaller element)
         Arrays.fill(result, -1);
 
+        // Traverse the array from left to right
         for (int i = 0; i < array.length; i++) {
-            while (!stack.empty() && stack.peek() >= array[i]) {
+            // Maintain the stack such that the top of the stack is the next smaller element
+            while (!stack.isEmpty() && stack.peek() >= array[i]) {
                 stack.pop();
             }
-            if (stack.empty()) {
-                result[i] = -1;
-            } else {
+
+            // If stack is not empty, then the top is the next smaller element
+            if (!stack.isEmpty()) {
                 result[i] = stack.peek();
             }
+
+            // Push the current element onto the stack
             stack.push(array[i]);
         }
-        return result;
-    }
 
-    public static void main(String[] args) {
-        int[] input = {2, 7, 3, 5, 4, 6, 8};
-        int[] result = findNextSmallerElements(input);
-        System.out.println(Arrays.toString(result));
+        return result;
     }
 }

src/main/java/com/thealgorithms/strings/ReverseStringRecursive.java

@@ -3,10 +3,10 @@
 /**
  * Reverse String using Recursion
  */
-
 public final class ReverseStringRecursive {
     private ReverseStringRecursive() {
     }
+
     /**
      * @param str string to be reversed
      * @return reversed string

src/main/java/com/thealgorithms/strings/StringCompression.java

@@ -1,7 +1,8 @@
 package com.thealgorithms.strings;
-/*  References : https://en.wikipedia.org/wiki/Run-length_encoding
- * String compression algorithm deals with encoding the string, that is, shortening the size of the
- * string
+
+/**
+ * References : https://en.wikipedia.org/wiki/Run-length_encoding
+ * String compression algorithm deals with encoding the string, that is, shortening the size of the string
  * @author Swarga-codes (https://github.com/Swarga-codes)
  */
 public final class StringCompression {
@@ -14,19 +15,16 @@ private StringCompression() {
      * @return the compressed character array as string
      */
     public static String compress(String input) {
-        // Keeping the count as 1 since every element present will have atleast a count
-        // of 1
+        // Keeping the count as 1 since every element present will have at least a count of 1
         int count = 1;
         String compressedString = "";
-        // Base condition to check whether the array is of size 1, if it is then we
-        // return the array
+        // Base condition to check whether the array is of size 1, if it is then we return the array
         if (input.length() == 1) {
             return "" + input.charAt(0);
         }
         // If the array has a length greater than 1 we move into this loop
         for (int i = 0; i < input.length() - 1; i++) {
-            // here we check for similarity of the adjacent elements and change the count
-            // accordingly
+            // here we check for similarity of the adjacent elements and change the count accordingly
             if (input.charAt(i) == input.charAt(i + 1)) {
                 count = count + 1;
             }
@@ -54,7 +52,6 @@ public static String compress(String input) {
     public static String appendCount(String res, int count, char ch) {
         if (count > 1) {
             res += ch + "" + count;
-            count = 1;
         } else {
             res += ch + "";
         }

src/test/java/com/thealgorithms/stacks/NextGreaterElementTest.java

@@ -0,0 +1,29 @@
+package com.thealgorithms.stacks;
+
+import static org.junit.jupiter.api.Assertions.assertArrayEquals;
+import static org.junit.jupiter.api.Assertions.assertThrows;
+
+import java.util.stream.Stream;
+import org.junit.jupiter.api.Test;
+import org.junit.jupiter.params.ParameterizedTest;
+import org.junit.jupiter.params.provider.Arguments;
+import org.junit.jupiter.params.provider.MethodSource;
+
+class NextGreaterElementTest {
+
+    @ParameterizedTest
+    @MethodSource("provideTestCases")
+    void testFindNextGreaterElements(int[] input, int[] expected) {
+        assertArrayEquals(expected, NextGreaterElement.findNextGreaterElements(input));
+    }
+
+    static Stream<Arguments> provideTestCases() {
+        return Stream.of(Arguments.of(new int[] {2, 7, 3, 5, 4, 6, 8}, new int[] {7, 8, 5, 6, 6, 8, 0}), Arguments.of(new int[] {5}, new int[] {0}), Arguments.of(new int[] {1, 2, 3, 4, 5}, new int[] {2, 3, 4, 5, 0}), Arguments.of(new int[] {5, 4, 3, 2, 1}, new int[] {0, 0, 0, 0, 0}),
+            Arguments.of(new int[] {4, 5, 2, 25}, new int[] {5, 25, 25, 0}), Arguments.of(new int[] {}, new int[] {}));
+    }
+
+    @Test
+    void testNullInput() {
+        assertThrows(IllegalArgumentException.class, () -> NextGreaterElement.findNextGreaterElements(null));
+    }
+}

src/test/java/com/thealgorithms/stacks/NextSmallerElementTest.java

@@ -0,0 +1,29 @@
+package com.thealgorithms.stacks;
+
+import static org.junit.jupiter.api.Assertions.assertArrayEquals;
+import static org.junit.jupiter.api.Assertions.assertThrows;
+
+import java.util.stream.Stream;
+import org.junit.jupiter.api.Test;
+import org.junit.jupiter.params.ParameterizedTest;
+import org.junit.jupiter.params.provider.Arguments;
+import org.junit.jupiter.params.provider.MethodSource;
+
+class NextSmallerElementTest {
+
+    @ParameterizedTest
+    @MethodSource("provideTestCases")
+    void testFindNextSmallerElements(int[] input, int[] expected) {
+        assertArrayEquals(expected, NextSmallerElement.findNextSmallerElements(input));
+    }
+
+    static Stream<Arguments> provideTestCases() {
+        return Stream.of(Arguments.of(new int[] {2, 7, 3, 5, 4, 6, 8}, new int[] {-1, 2, 2, 3, 3, 4, 6}), Arguments.of(new int[] {5}, new int[] {-1}), Arguments.of(new int[] {1, 2, 3, 4, 5}, new int[] {-1, 1, 2, 3, 4}), Arguments.of(new int[] {5, 4, 3, 2, 1}, new int[] {-1, -1, -1, -1, -1}),
+            Arguments.of(new int[] {4, 5, 2, 25}, new int[] {-1, 4, -1, 2}), Arguments.of(new int[] {}, new int[] {}));
+    }
+
+    @Test
+    void testFindNextSmallerElementsExceptions() {
+        assertThrows(IllegalArgumentException.class, () -> NextSmallerElement.findNextSmallerElements(null));
+    }
+}

src/test/java/com/thealgorithms/strings/ReverseStringRecursiveTest.java

@@ -2,30 +2,15 @@
 
 import static org.junit.jupiter.api.Assertions.assertEquals;
 
-import org.junit.jupiter.api.Test;
+import org.junit.jupiter.params.ParameterizedTest;
+import org.junit.jupiter.params.provider.CsvSource;
 
 public class ReverseStringRecursiveTest {
-    @Test
-    void shouldAcceptWhenEmptyStringIsPassed() {
-        String expected = "";
-        String reversed = ReverseStringRecursive.reverse("");
-
-        assertEquals(expected, reversed);
-    }
-
-    @Test
-    void shouldAcceptNotWhenWhenSingleCharacterIsPassed() {
-        String expected = "a";
-        String reversed = ReverseStringRecursive.reverse("a");
-
-        assertEquals(expected, reversed);
-    }
-
-    @Test
-    void shouldAcceptWhenStringIsPassed() {
-        String expected = "dlroWolleH";
-        String reversed = ReverseStringRecursive.reverse("HelloWorld");
-
-        assertEquals(expected, reversed);
+    @ParameterizedTest
+    @CsvSource({"'Hello World', 'dlroW olleH'", "'helloworld', 'dlrowolleh'", "'123456789', '987654321'", "'', ''", "'A', 'A'", "'!123 ABC xyz!', '!zyx CBA 321!'", "'Abc 123 Xyz', 'zyX 321 cbA'", "'12.34,56;78:90', '09:87;65,43.21'", "'abcdEFGHiJKL', 'LKJiHGFEdcba'",
+        "'MixOf123AndText!', '!txeTdnA321fOxiM'"})
+    public void
+    testReverseString(String input, String expectedOutput) {
+        assertEquals(expectedOutput, ReverseStringRecursive.reverse(input));
     }
 }

src/test/java/com/thealgorithms/strings/StringCompressionTest.java

@@ -1,12 +1,13 @@
 package com.thealgorithms.strings;
+
 import static org.junit.jupiter.api.Assertions.assertEquals;
 
 import org.junit.jupiter.params.ParameterizedTest;
 import org.junit.jupiter.params.provider.CsvSource;
 
 public class StringCompressionTest {
     @ParameterizedTest
-    @CsvSource({"a,a", "aabbb,a2b3", "abbbc,ab3c", "aabccd,a2bc2d"})
+    @CsvSource({"'a', 'a'", "'aabbb', 'a2b3'", "'abbbc', 'ab3c'", "'aabccd', 'a2bc2d'", "'aaaabbbcccc', 'a4b3c4'", "'abcd', 'abcd'", "'aabbccdd', 'a2b2c2d2'", "'aaabbaa', 'a3b2a2'", "'', ''", "'a', 'a'", "'aaaaa', 'a5'", "'aabb', 'a2b2'", "'aabbbaaa', 'a2b3a3'", "'qwerty', 'qwerty'"})
     void stringCompressionTest(String input, String expectedOutput) {
         String output = StringCompression.compress(input);
         assertEquals(expectedOutput, output);