feat(strings): add MoveHashToEnd algorithm (#7313)

* feat(strings): add MoveHashToEnd algorithm with tests

- Add MoveHashToEnd utility class that moves all '#' characters
  to the end of a string while preserving the order of other characters
- Algorithm runs in O(n) time and O(n) space using a two-pass approach:
  first collects non-'#' chars, then fills remaining positions with '#'
- Add null and empty string guards
- Add MoveHashToEndTest with 9 unit tests covering normal, edge,
  and boundary cases (null, empty, all-hash, no-hash, single char)

* docs(strings): add reference URL to MoveHashToEnd Javadoc

* docs: add MoveHashToEnd to DIRECTORY index

* style(strings): add missing newline at EOF in MoveHashToEnd

* test(strings): fix MoveHashToEnd expected output for sample input

Author: ShyamChavda005

DIRECTORY.md

@@ -816,6 +816,7 @@
           - 📄 [Lower](src/main/java/com/thealgorithms/strings/Lower.java)
           - 📄 [Manacher](src/main/java/com/thealgorithms/strings/Manacher.java)
           - 📄 [MyAtoi](src/main/java/com/thealgorithms/strings/MyAtoi.java)
+          - 📄 [MoveHashToEnd](src/main/java/com/thealgorithms/strings/MoveHashToEnd.java)
           - 📄 [Palindrome](src/main/java/com/thealgorithms/strings/Palindrome.java)
           - 📄 [Pangram](src/main/java/com/thealgorithms/strings/Pangram.java)
           - 📄 [PermuteString](src/main/java/com/thealgorithms/strings/PermuteString.java)

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

@@ -0,0 +1,56 @@
+package com.thealgorithms.strings;
+
+/**
+ * Moves all '#' characters to the end of the given string while preserving
+ * the order of the other characters.
+ *
+ * Example:
+ * Input  : "h#e#l#llo"
+ * Output : "helllo###"
+ *
+ * The algorithm works by iterating through the string and collecting
+ * all non-# characters first, then filling the remaining positions
+ * with '#'.
+ *
+ * Time Complexity: O(n)
+ * Space Complexity: O(n)
+ *
+ * @see <a href="https://www.geeksforgeeks.org/move-special-char-end-string-maintain-order-alphabets/">Move all special characters to end - GeeksForGeeks</a>
+ */
+public final class MoveHashToEnd {
+
+    /**
+     * Private constructor to prevent instantiation of utility class.
+     */
+    private MoveHashToEnd() {
+    }
+
+    /**
+     * Moves all '#' characters in the input string to the end.
+     *
+     * @param str the input string containing characters and '#'
+     * @return a new string with all '#' characters moved to the end
+     */
+    public static String moveHashToEnd(String str) {
+        if (str == null || str.isEmpty()) {
+            return str;
+        }
+
+        char[] arr = str.toCharArray();
+        int insertPos = 0;
+
+        // Place all non-# characters at the beginning
+        for (char ch : arr) {
+            if (ch != '#') {
+                arr[insertPos++] = ch;
+            }
+        }
+
+        // Fill remaining positions with '#'
+        while (insertPos < arr.length) {
+            arr[insertPos++] = '#';
+        }
+
+        return new String(arr);
+    }
+}

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

@@ -0,0 +1,54 @@
+package com.thealgorithms.strings;
+
+import static org.junit.jupiter.api.Assertions.assertEquals;
+import static org.junit.jupiter.api.Assertions.assertNull;
+
+import org.junit.jupiter.api.Test;
+
+public class MoveHashToEndTest {
+
+    @Test
+    void testBasicCase() {
+        assertEquals("helllo###", MoveHashToEnd.moveHashToEnd("h#e#l#llo"));
+    }
+
+    @Test
+    void testNoHash() {
+        assertEquals("hello", MoveHashToEnd.moveHashToEnd("hello"));
+    }
+
+    @Test
+    void testAllHashes() {
+        assertEquals("###", MoveHashToEnd.moveHashToEnd("###"));
+    }
+
+    @Test
+    void testHashAtEnd() {
+        assertEquals("hello#", MoveHashToEnd.moveHashToEnd("hello#"));
+    }
+
+    @Test
+    void testHashAtStart() {
+        assertEquals("hello#", MoveHashToEnd.moveHashToEnd("#hello"));
+    }
+
+    @Test
+    void testEmptyString() {
+        assertEquals("", MoveHashToEnd.moveHashToEnd(""));
+    }
+
+    @Test
+    void testNullInput() {
+        assertNull(MoveHashToEnd.moveHashToEnd(null));
+    }
+
+    @Test
+    void testSingleHash() {
+        assertEquals("#", MoveHashToEnd.moveHashToEnd("#"));
+    }
+
+    @Test
+    void testSingleNonHashChar() {
+        assertEquals("a", MoveHashToEnd.moveHashToEnd("a"));
+    }
+}