added palindrome checker using stack

Author: S-M-J-I

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

@@ -0,0 +1,59 @@
+package com.thealgorithms.stacks;
+
+import java.util.LinkedList;
+
+/**
+ * A class that implements a palindrome checker using a stack.
+ * The stack is used to store the characters of the string,
+ * which we will pop one-by-one to create the string in reverse.
+ *
+ * Reference: https://www.geeksforgeeks.org/check-whether-the-given-string-is-palindrome-using-stack/
+ */
+public final class PalindromeWithStack {
+    private LinkedList<Character> stack;
+
+    /**
+     * Constructs an empty stack that stores characters.
+     */
+    public PalindromeWithStack() {
+        stack = new LinkedList<Character>();
+    }
+
+    /**
+     * Check if the string is a palindrome or not.
+     * Convert all characters to lowercase and push them into a stack.
+     * At the same time, build a string
+     * Next, pop from the stack and build the reverse string
+     * Finally, compare these two strings
+     *
+     * @param string The string to check if it is palindrome or not.
+     */
+    public final boolean checkPalindrome(final String string) {
+        // Create a StringBuilder to build the string from left to right
+        StringBuilder stringBuilder = new StringBuilder(string.length());
+        // Convert all characters to lowercase
+        String lowercase = string.toLowerCase();
+
+        // Iterate through the string
+        for(int i = 0; i < lowercase.length(); ++i) {
+            char c = lowercase.charAt(i);
+            if (c >= 'a' && c <= 'z') {
+                // Build the string from L->R
+                stringBuilder.append(c);
+                // Push to the stack
+                stack.push(c);
+            }
+        }
+
+        // The stack contains the reverse order of the string
+        StringBuilder reverseString = new StringBuilder(stack.size());
+        // Until the stack is not empty
+        while (!stack.isEmpty()){
+            // Build the string from R->L
+            reverseString.append(stack.pop());
+        }
+
+        // Finally, compare the L->R string with the R->L string
+        return reverseString.toString().equals(stringBuilder.toString());
+    }
+}

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

@@ -0,0 +1,59 @@
+package com.thealgorithms.stacks;
+
+import static org.junit.jupiter.api.Assertions.assertFalse;
+import static org.junit.jupiter.api.Assertions.assertTrue;
+
+import org.junit.jupiter.api.BeforeEach;
+import org.junit.jupiter.api.Test;
+
+public class PalindromeWithStackTest {
+
+    private PalindromeWithStack palindromeChecker;
+
+    @BeforeEach
+    public void setUp() {
+        palindromeChecker = new PalindromeWithStack();
+    }
+
+    @Test
+    public void testValidOne() {
+        String testString = "Racecar";
+        assertTrue(palindromeChecker.checkPalindrome(testString));
+    }
+
+    @Test
+    public void testInvalidOne() {
+        String testString = "James";
+        assertFalse(palindromeChecker.checkPalindrome(testString));
+    }
+
+    @Test
+    public void testValidTwo() {
+        String testString = "madam";
+        assertTrue(palindromeChecker.checkPalindrome(testString));
+    }
+
+    @Test
+    public void testInvalidTwo() {
+        String testString = "pantry";
+        assertFalse(palindromeChecker.checkPalindrome(testString));
+    }
+
+    @Test
+    public void testValidThree() {
+        String testString = "RaDar";
+        assertTrue(palindromeChecker.checkPalindrome(testString));
+    }
+
+    @Test
+    public void testInvalidThree() {
+        String testString = "Win";
+        assertFalse(palindromeChecker.checkPalindrome(testString));
+    }
+
+    @Test
+    public void testBlankString() {
+        String testString = "";
+        assertTrue(palindromeChecker.checkPalindrome(testString));
+    }
+}