diff --git a/DataStructures/Lists/SinglyLinkedList.java b/DataStructures/Lists/SinglyLinkedList.java
index 8747aebb6..a478d7a5f 100644
--- a/DataStructures/Lists/SinglyLinkedList.java
+++ b/DataStructures/Lists/SinglyLinkedList.java
@@ -54,20 +54,16 @@ public class SinglyLinkedList {
      * @param data     data to be stored in a new node
      * @param position position at which a new node is to be inserted
      */
-
     public void insertNth(int data, int position) {
-        if (position < 0 || position > getSize()) {
-            throw new IndexOutOfBoundsException("position less than zero or position more than the count of list");
-        } else {
-            Node cur = head;
-            Node node = new Node(data);
-            for (int i = 0; i < position; ++i) {
-                cur = cur.next;
-            }
-            node.next = cur.next;
-            cur.next = node;
-            size++;
+        checkBounds(position, 0, size);
+        Node cur = head;
+        for (int i = 0; i < position; ++i) {
+            cur = cur.next;
         }
+        Node node = new Node(data);
+        node.next = cur.next;
+        cur.next = node;
+        size++;
     }
 
     /**
@@ -90,20 +86,48 @@ public class SinglyLinkedList {
      * This method deletes an element at Nth position
      */
     public void deleteNth(int position) {
-        if (position < 0 || position > size - 1) {
-            throw new IndexOutOfBoundsException("position less than zero or position more than the count of list");
-        } else {
-            Node cur = head;
-            for (int i = 0; i < position; ++i) {
-                cur = cur.next;
-            }
-
-            Node destroy = cur.next;
-            cur.next = cur.next.next;
-            destroy = null; // clear to let GC do its work
-
-            size--;
+        checkBounds(position, 0, size - 1);
+        Node cur = head;
+        for (int i = 0; i < position; ++i) {
+            cur = cur.next;
         }
+
+        Node destroy = cur.next;
+        cur.next = cur.next.next;
+        destroy = null; // clear to let GC do its work
+
+        size--;
+    }
+
+    /**
+     * @param position to check position
+     * @param low      low index
+     * @param high     high index
+     * @throws IndexOutOfBoundsException if {@code position} not in range {@code low} to {@code high}
+     */
+    public void checkBounds(int position, int low, int high) {
+        if (position < low || position > high) {
+            throw new IndexOutOfBoundsException(position + "");
+        }
+    }
+
+    /**
+     * clear all nodes in list
+     */
+    public void clear() {
+        if (size == 0) {
+            return;
+        }
+        Node prev = head.next;
+        Node cur = prev.next;
+        while (cur != null) {
+            prev = null; // clear to let GC do its work
+            prev = cur;
+            cur = cur.next;
+        }
+        prev = null;
+        head.next = null;
+        size = 0;
     }
 
     /**
@@ -115,18 +139,6 @@ public class SinglyLinkedList {
         return size == 0;
     }
 
-    /**
-     * Prints contents of the list
-     */
-    public void display() {
-        Node current = head.next;
-        while (current != null) {
-            System.out.print(current.value + " ");
-            current = current.next;
-        }
-        System.out.println();
-    }
-
     /**
      * Returns the size of the linked list
      */
@@ -134,6 +146,20 @@ public class SinglyLinkedList {
         return size;
     }
 
+    @Override
+    public String toString() {
+        if (size == 0) {
+            return "";
+        }
+        StringBuilder builder = new StringBuilder();
+        Node cur = head.next;
+        while (cur != null) {
+            builder.append(cur.value).append("->");
+            cur = cur.next;
+        }
+        return builder.replace(builder.length() - 2, builder.length(), "").toString();
+    }
+
     /**
      * Main method
      *
@@ -148,19 +174,24 @@ public class SinglyLinkedList {
         myList.insertHead(7);
         myList.insertHead(10);
 
-        myList.display(); // 10 -> 7 -> 5
+        System.out.println(myList);; // 10 -> 7 -> 5
 
         myList.deleteHead();
 
-        myList.display(); // 7 -> 5
+        System.out.println(myList);; // 7 -> 5
 
         myList.insertNth(11, 2);
 
-        myList.display(); // 7 -> 5 -> 11
+        System.out.println(myList);; // 7 -> 5 -> 11
 
         myList.deleteNth(1);
 
-        myList.display(); // 7-> 11
+        System.out.println(myList);; // 7-> 11
+
+        myList.clear();
+        assert myList.isEmpty();
+
+        System.out.println(myList); // ""
 
     }
 }
diff --git a/DataStructures/Stacks/BalancedBrackets.java b/DataStructures/Stacks/BalancedBrackets.java
index 8063c5105..ea342219c 100644
--- a/DataStructures/Stacks/BalancedBrackets.java
+++ b/DataStructures/Stacks/BalancedBrackets.java
@@ -1,10 +1,8 @@
-package data_structures.Stacks;
+package DataStructures.Stacks;
 
-import java.util.Scanner;
 import java.util.Stack;
 
 /**
- *
  * The nested brackets problem is a problem that determines if a sequence of
  * brackets are properly nested. A sequence of brackets s is considered properly
  * nested if any of the following conditions are true: - s is empty - s has the
@@ -15,71 +13,69 @@ import java.util.Stack;
  * returns true if S is nested and false otherwise.
  *
  * @author akshay sharma
- * @date: 2017-10-17
  * @author <a href="https://github.com/khalil2535">khalil2535<a>
- *
+ * @author shellhub
  */
 class BalancedBrackets {
 
     /**
+     * Check if {@code leftBracket} and {@code rightBracket} is paired or not
      *
-     * @param s
-     * @return
+     * @param leftBracket   left bracket
+     * @param rightBracket right bracket
+     * @return {@code true} if {@code leftBracket} and {@code rightBracket} is paired,
+     * otherwise {@code false}
      */
-    static boolean is_balanced(String s) {
-        Stack<Character> bracketsStack = new Stack<>();
-        char[] text = s.toCharArray();
-        for (char x : text) {
-            switch (x) {
-                case '{':
-                case '<':
-                case '(':
-                case '[':
-                    bracketsStack.push(x);
-                    break;
-                case '}':
-                    if (!bracketsStack.empty() && bracketsStack.pop() == '{') {
-                        break;
-                    } else {
-                        return false;
-                    }
-                case '>':
-                    if (!bracketsStack.empty() && bracketsStack.pop() == '<') {
-                        break;
-                    } else {
-                        return false;
-                    }
-                case ')':
-                    if (!bracketsStack.empty() && bracketsStack.pop() == '(') {
-                        break;
-                    } else {
-                        return false;
-                    }
-                case ']':
-                    if (!bracketsStack.empty() && bracketsStack.pop() == '[') {
-                        break;
-                    } else {
-                        return false;
-                    }
+    public static boolean isPaired(char leftBracket, char rightBracket) {
+        char[][] pairedBrackets = {
+                {'(', ')'},
+                {'[', ']'},
+                {'{', '}'},
+                {'<', '>'}
+        };
+        for (char[] pairedBracket : pairedBrackets) {
+            if (pairedBracket[0] == leftBracket && pairedBracket[1] == rightBracket) {
+                return true;
             }
         }
-        return bracketsStack.empty();
+        return false;
     }
 
     /**
+     * Check if {@code brackets} is balanced
      *
-     * @param args
-     * @TODO remove main method and Test using JUnit or other methodology
+     * @param brackets the brackets
+     * @return {@code true} if {@code brackets} is balanced, otherwise {@code false}
      */
-    public static void main(String args[]) {
-        try (Scanner in = new Scanner(System.in)) {
-            System.out.println("Enter sequence of brackets: ");
-            String s = in.nextLine();
-            if (is_balanced(s)) {
-                System.out.println(s + " is balanced");
-            } else {
-                System.out.println(s + " ain't balanced");
+    public static boolean isBalanced(String brackets) {
+        if (brackets == null) {
+            throw new IllegalArgumentException("brackets is null");
+        }
+        Stack<Character> bracketsStack = new Stack<>();
+        for (char bracket : brackets.toCharArray()) {
+            switch (bracket) {
+                case '(':
+                case '[':
+                case '{':
+                    bracketsStack.push(bracket);
+                    break;
+                case ')':
+                case ']':
+                case '}':
+                    if (!(!bracketsStack.isEmpty() && isPaired(bracketsStack.pop(), bracket))) {
+                        return false;
+                    }
+                    break;
+                default: /* other character is invalid */
+                    return false;
             }
         }
+        return bracketsStack.isEmpty();
+    }
+
+
+    public static void main(String[] args) {
+        assert isBalanced("[()]{}{[()()]()}");
+        assert !isBalanced("[(])");
     }
 }
diff --git a/DataStructures/Stacks/DecimalToAnyUsingStack.java b/DataStructures/Stacks/DecimalToAnyUsingStack.java
new file mode 100644
index 000000000..6e129c32c
--- /dev/null
+++ b/DataStructures/Stacks/DecimalToAnyUsingStack.java
@@ -0,0 +1,44 @@
+package DataStructures.Stacks;
+
+import java.util.Stack;
+
+public class DecimalToAnyUsingStack {
+    public static void main(String[] args) {
+        assert convert(0, 2).equals("0");
+        assert convert(30, 2).equals("11110");
+        assert convert(30, 8).equals("36");
+        assert convert(30, 10).equals("30");
+        assert convert(30, 16).equals("1E");
+    }
+
+    /**
+     * Convert decimal number to another radix
+     *
+     * @param number the number to be converted
+     * @param radix the radix
+     * @return another radix
+     * @throws ArithmeticException if <tt>number</tt> or <tt>radius</tt> is invalid
+     */
+    private static String convert(int number, int radix) {
+        if (radix < 2 || radix > 16) {
+            throw new ArithmeticException(
+                    String.format("Invalid input -> number:%d,radius:%d", number, radix));
+        }
+        char[] tables = {
+                '0', '1', '2', '3', '4',
+                '5', '6', '7', '8', '9',
+                'A', 'B', 'C', 'D', 'E', 'F'
+        };
+        Stack<Character> bits = new Stack<>();
+        do {
+            bits.push(tables[number % radix]);
+            number = number / radix;
+        } while (number != 0);
+
+        StringBuilder result = new StringBuilder();
+        while (!bits.isEmpty()) {
+            result.append(bits.pop());
+        }
+        return result.toString();
+    }
+}
diff --git a/DataStructures/Stacks/StackOfLinkedList.java b/DataStructures/Stacks/StackOfLinkedList.java
index 093442112..5a2b8fa08 100644
--- a/DataStructures/Stacks/StackOfLinkedList.java
+++ b/DataStructures/Stacks/StackOfLinkedList.java
@@ -17,12 +17,12 @@ class StackOfLinkedList {
         stack.push(4);
         stack.push(5);
 
-        stack.printStack();
+        System.out.println(stack);
 
         System.out.println("Size of stack currently is: " + stack.getSize());
 
-        stack.pop();
-        stack.pop();
+        assert stack.pop() == 5;
+        assert stack.pop() == 4;
 
         System.out.println("Top element of stack currently is: " + stack.peek());
     }
@@ -48,65 +48,95 @@ class Node {
 
 class LinkedListStack {
 
-    Node head = null;
+    /**
+     * Top of stack
+     */
+    Node head;
 
-    public void push(int x) {
-        Node n = new Node(x);
-        if (head == null) {
-            head = n;
-        } else {
-            Node temp = head;
-            n.next = temp;
-            head = n;
-        }
+    /**
+     * Size of stack
+     */
+    private int size;
+
+    /**
+     * Init properties
+     */
+    public LinkedListStack() {
+        head = null;
+        size = 0;
     }
 
-    public void pop() {
-        if (head == null) {
-            System.out.println("Empty stack. Nothing to pop");
-        }
+    /**
+     * Add element at top
+     *
+     * @param x to be added
+     * @return <tt>true</tt> if add successfully
+     */
+    public boolean push(int x) {
+        Node newNode = new Node(x);
+        newNode.next = head;
+        head = newNode;
+        size++;
+        return true;
+    }
 
-        Node temp = head;
+    /**
+     * Pop element at top of stack
+     *
+     * @return element at top of stack
+     * @throws NoSuchElementException if stack is empty
+     */
+    public int pop() {
+        if (size == 0) {
+            throw new NoSuchElementException("Empty stack. Nothing to pop");
+        }
+        Node destroy = head;
         head = head.next;
-        System.out.println("Popped element is: " + temp.data);
+        int retValue = destroy.data;
+        destroy = null; // clear to let GC do it's work
+        size--;
+        return retValue;
     }
 
+    /**
+     * Peek element at top of stack
+     *
+     * @return element at top of stack
+     * @throws NoSuchElementException if stack is empty
+     */
     public int peek() {
-        if (head == null) {
-            return -1;
+        if (size == 0) {
+            throw new NoSuchElementException("Empty stack. Nothing to pop");
         }
         return head.data;
     }
 
-    public void printStack() {
-        Node temp = head;
-        System.out.println("Stack is printed as below: ");
-        while (temp != null) {
-            if (temp.next == null) {
-                System.out.print(temp.data);
-            } else {
-                System.out.print(temp.data + " -> ");
-            }
-            temp = temp.next;
+    @Override
+    public String toString() {
+        Node cur = head;
+        StringBuilder builder = new StringBuilder();
+        while (cur != null) {
+            builder.append(cur.data).append("->");
+            cur = cur.next;
         }
-        System.out.println();
+        return builder.replace(builder.length() - 2, builder.length(), "").toString();
     }
 
+    /**
+     * Check if stack is empty
+     *
+     * @return <tt>true</tt> if stack is empty, otherwise <tt>false</tt>
+     */
     public boolean isEmpty() {
-        return head == null;
+        return size == 0;
     }
 
+    /**
+     * Return size of stack
+     *
+     * @return size of stack
+     */
     public int getSize() {
-        if (head == null)
-            return 0;
-        else {
-            int size = 1;
-            Node temp = head;
-            while (temp.next != null) {
-                temp = temp.next;
-                size++;
-            }
-            return size;
-        }
+        return size;
     }
 }
diff --git a/DynamicProgramming/Fibonacci.java b/DynamicProgramming/Fibonacci.java
index ba77919f8..112a014a3 100644
--- a/DynamicProgramming/Fibonacci.java
+++ b/DynamicProgramming/Fibonacci.java
@@ -12,6 +12,7 @@ public class Fibonacci {
 
     private static Map<Integer, Integer> map = new HashMap<>();
 
+
     public static void main(String[] args) {
 
         // Methods all returning [0, 1, 1, 2, 3, 5, ...] for n = [0, 1, 2, 3, 4, 5, ...]
diff --git a/DynamicProgramming/RodCutting.java b/DynamicProgramming/RodCutting.java
index d57c69ce9..0913dfd80 100644
--- a/DynamicProgramming/RodCutting.java
+++ b/DynamicProgramming/RodCutting.java
@@ -26,7 +26,8 @@ public class RodCutting {
     public static void main(String args[]) {
         int[] arr = new int[]{2, 5, 13, 19, 20};
         int size = arr.length;
+	int result = cutRod(arr,size); 
         System.out.println("Maximum Obtainable Value is " +
-                cutRod(arr, size));
+                result);
     }
 }
diff --git a/Maths/FindMin.java b/Maths/FindMin.java
index 2a13fe4b5..9097c0179 100644
--- a/Maths/FindMin.java
+++ b/Maths/FindMin.java
@@ -9,7 +9,7 @@ public class FindMin {
     }
 
     /**
-     * find min of array
+     * Find the minimum number of an array of numbers.
      *
      * @param array the array contains element
      * @return min value
diff --git a/Maths/PalindromeNumber.java b/Maths/PalindromeNumber.java
new file mode 100644
index 000000000..2916c753e
--- /dev/null
+++ b/Maths/PalindromeNumber.java
@@ -0,0 +1,30 @@
+package Maths;
+
+public class PalindromeNumber {
+    public static void main(String[] args) {
+
+        assert isPalindrome(12321);
+        assert !isPalindrome(1234);
+        assert isPalindrome(1);
+    }
+
+    /**
+     * Check if {@code n} is palindrome number or not
+     *
+     * @param number the number
+     * @return {@code true} if {@code n} is palindrome number, otherwise {@code false}
+     */
+    public static boolean isPalindrome(int number) {
+        if (number < 0) {
+            throw new IllegalArgumentException(number + "");
+        }
+        int numberCopy = number;
+        int reverseNumber = 0;
+        while (numberCopy != 0) {
+            int remainder = numberCopy % 10;
+            reverseNumber = reverseNumber * 10 + remainder;
+            numberCopy /= 10;
+        }
+        return number == reverseNumber;
+    }
+}
diff --git a/Maths/ParseInteger.java b/Maths/ParseInteger.java
new file mode 100644
index 000000000..91177bb49
--- /dev/null
+++ b/Maths/ParseInteger.java
@@ -0,0 +1,33 @@
+package Maths;
+
+public class ParseInteger {
+    public static void main(String[] args) {
+        assert parseInt("123") == Integer.parseInt("123");
+        assert parseInt("-123") == Integer.parseInt("-123");
+        assert parseInt("0123") == Integer.parseInt("0123");
+        assert parseInt("+123") == Integer.parseInt("+123");
+    }
+
+    /**
+     * Parse a string to integer
+     *
+     * @param s the string
+     * @return the integer value represented by the argument in decimal.
+     * @throws NumberFormatException if the {@code string} does not contain a parsable integer.
+     */
+    public static int parseInt(String s) {
+        if (s == null) {
+            throw new NumberFormatException("null");
+        }
+        boolean isNegative = s.charAt(0) == '-';
+        boolean isPositive = s.charAt(0) == '+';
+        int number = 0;
+        for (int i = isNegative ? 1 : isPositive ? 1 : 0, length = s.length(); i < length; ++i) {
+            if (!Character.isDigit(s.charAt(i))) {
+                throw new NumberFormatException("s=" + s);
+            }
+            number = number * 10 + s.charAt(i) - '0';
+        }
+        return isNegative ? -number : number;
+    }
+}
diff --git a/Maths/PerfectNumber.java b/Maths/PerfectNumber.java
new file mode 100644
index 000000000..ceaf0b2bc
--- /dev/null
+++ b/Maths/PerfectNumber.java
@@ -0,0 +1,33 @@
+package Maths;
+
+/**
+ * In number theory, a perfect number is a positive integer that is equal to the sum of
+ * its positive divisors, excluding the number itself. For instance, 6 has divisors 1, 2 and 3
+ * (excluding itself), and 1 + 2 + 3 = 6, so 6 is a perfect number.
+ * <p>
+ * link:https://en.wikipedia.org/wiki/Perfect_number
+ * </p>
+ */
+public class PerfectNumber {
+    public static void main(String[] args) {
+        assert isPerfectNumber(6); /* 1 + 2 + 3 == 6 */
+        assert !isPerfectNumber(8); /* 1 + 2 + 4 != 8 */
+        assert isPerfectNumber(28); /* 1 + 2 + 4 + 7 + 14 == 28 */
+    }
+
+    /**
+     * Check if {@code number} is perfect number or not
+     *
+     * @param number the number
+     * @return {@code true} if {@code number} is perfect number, otherwise false
+     */
+    public static boolean isPerfectNumber(int number) {
+        int sum = 0;  /* sum of its positive divisors */
+        for (int i = 1; i < number; ++i) {
+            if (number % i == 0) {
+                sum += i;
+            }
+        }
+        return sum == number;
+    }
+}
diff --git a/Maths/Pow.java b/Maths/Pow.java
new file mode 100644
index 000000000..605e01d98
--- /dev/null
+++ b/Maths/Pow.java
@@ -0,0 +1,26 @@
+package maths;
+
+public class Pow {
+    public static void main(String[] args) {
+        assert pow(2, 0) == Math.pow(2, 0);
+        assert pow(0, 2) == Math.pow(0, 2);
+        assert pow(2, 10) == Math.pow(2, 10);
+        assert pow(10, 2) == Math.pow(10, 2);
+    }
+
+    /**
+     * Returns the value of the first argument raised to the power of the
+     * second argument
+     *
+     * @param a the base.
+     * @param b the exponent.
+     * @return the value {@code a}<sup>{@code b}</sup>.
+     */
+    public static long pow(int a, int b) {
+        long result = 1;
+        for (int i = 1; i <= b; i++) {
+            result *= a;
+        }
+        return result;
+    }
+}
diff --git a/Maths/PowRecursion.java b/Maths/PowRecursion.java
new file mode 100644
index 000000000..243548708
--- /dev/null
+++ b/Maths/PowRecursion.java
@@ -0,0 +1,26 @@
+package Maths;
+
+public class PowRecursion {
+    public static void main(String[] args) {
+        assert pow(2, 0) == Math.pow(2, 0);
+        assert pow(0, 2) == Math.pow(0, 2);
+        assert pow(2, 10) == Math.pow(2, 10);
+        assert pow(10, 2) == Math.pow(10, 2);
+    }
+
+    /**
+     * Returns the value of the first argument raised to the power of the
+     * second argument
+     *
+     * @param a the base.
+     * @param b the exponent.
+     * @return the value {@code a}<sup>{@code b}</sup>.
+     */
+    public static long pow(int a, int b) {
+        if (b == 0) {
+            return 1;
+        } else {
+            return a * pow(a, b - 1);
+        }
+    }
+}
diff --git a/Others/CountWords.java b/Others/CountWords.java
index 5dc637559..f1caf9a87 100644
--- a/Others/CountWords.java
+++ b/Others/CountWords.java
@@ -44,7 +44,5 @@ public class CountWords {
         }
         s = sb.toString();
         return s.trim().split("[\\s]+").length;
-
     }
-
 }