Implement LengthOfLastWord algorithm and add JUnit tests

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

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+package com.thealgorithms.strings;
+
+/**
+ * The {@code LengthOfLastWord} class provides a utility method to determine
+ * the length of the last word in a given string.
+ *
+ * <p>A "word" is defined as a maximal substring consisting of non-space
+ * characters only. Trailing spaces at the end of the string are ignored.
+ *
+ * <p><strong>Example:</strong>
+ * <pre>{@code
+ * LengthOfLastWord obj = new LengthOfLastWord();
+ * System.out.println(obj.lengthOfLastWord("Hello World"));  // Output: 5
+ * System.out.println(obj.lengthOfLastWord("  fly me   to   the moon  "));  // Output: 4
+ * System.out.println(obj.lengthOfLastWord("luffy is still joyboy"));  // Output: 6
+ * }</pre>
+ *
+ * <p>This implementation runs in O(n) time complexity, where n is the length
+ * of the input string, and uses O(1) additional space.
+ */
+public class LengthOfLastWord {
+
+    /**
+     * Returns the length of the last word in the specified string.
+     *
+     * <p>The method iterates from the end of the string, skipping trailing
+     * spaces first, and then counts the number of consecutive non-space characters
+     * characters until another space (or the beginning of the string) is reached.
+     *
+     * @param s the input string to analyze
+     * @return the length of the last word in {@code s}; returns 0 if there is no word
+     * @throws NullPointerException if {@code s} is {@code null}
+     */
+    public int lengthOfLastWord(String s) {
+        int sizeOfString = s.length() - 1;
+        int lastWordLength = 0;
+
+        // Skip trailing spaces from the end of the string
+        while (sizeOfString >= 0 && s.charAt(sizeOfString) == ' ') {
+            sizeOfString--;
+        }
+
+        // Count the characters of the last word
+        while (sizeOfString >= 0 && s.charAt(sizeOfString) != ' ') {
+            lastWordLength++;
+            sizeOfString--;
+        }
+
+        return lastWordLength;
+    }
+}

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

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+package com.thealgorithms.strings;
+
+import org.junit.jupiter.api.Test;
+
+import static org.junit.jupiter.api.Assertions.assertEquals;
+
+public class LengthOfLastWordTest {
+    @Test
+    public void testLengthOfLastWord() {
+        assertEquals(5, new LengthOfLastWord().lengthOfLastWord("Hello World"));
+        assertEquals(4, new LengthOfLastWord().lengthOfLastWord("  fly me   to   the moon  "));
+        assertEquals(6, new LengthOfLastWord().lengthOfLastWord("luffy is still joyboy"));
+        assertEquals(5, new LengthOfLastWord().lengthOfLastWord("Hello"));
+        assertEquals(0, new LengthOfLastWord().lengthOfLastWord("     "));
+        assertEquals(0, new LengthOfLastWord().lengthOfLastWord(""));
+        assertEquals(3, new LengthOfLastWord().lengthOfLastWord("JUST LIE  "));
+    }
+}