diff --git a/code/src/com/allendowney/thinkdast/N3Array.java b/code/src/com/allendowney/thinkdast/N3Array.java
new file mode 100644
index 00000000..593c4537
--- /dev/null
+++ b/code/src/com/allendowney/thinkdast/N3Array.java
@@ -0,0 +1,41 @@
+import java.util.*;
+import java.util.Map.Entry;
+public class N3Array {
+	public static void n3array(int arr[])
+	{
+		int n=arr.length;
+		Map<Integer,Integer>hm=new HashMap<>();
+		int count=0;
+		for(int i=0;i<arr.length;i++)
+		{
+			if(!hm.containsKey(arr[i]))
+			{
+				hm.put(arr[i],1);
+			}
+			else 
+			{
+				if(hm.get(arr[i])>=1)
+				{
+					count=hm.get(arr[i]);
+					hm.put(arr[i],count+1);
+				}
+			}
+		}
+		Iterator<Map.Entry<Integer,Integer>>it=hm.entrySet().iterator();
+		while(it.hasNext())
+		{ 
+			Entry<Integer, Integer> entry=it.next();
+			if(entry.getValue()>(n/3))
+			{
+				System.out.println(entry.getKey());
+			}
+			
+	}
+		
+	}
+public static void main(String []args)
+{
+	int arr[]= {3, 3, 4, 2, 4, 4, 2, 4, 4};
+	n3array(arr);
+}
+}
diff --git a/code/src/com/allendowney/thinkdast/NextPermutation.java b/code/src/com/allendowney/thinkdast/NextPermutation.java
new file mode 100644
index 00000000..5ec6ad92
--- /dev/null
+++ b/code/src/com/allendowney/thinkdast/NextPermutation.java
@@ -0,0 +1,34 @@
+import java.util.*;
+public class NextPermutation {
+	public static void permute(int a[])
+	{
+		int n=a.length;
+		for(int i=n-1;i>0;i-- )
+		{
+			if(a[i]>a[i-1])
+			{
+			int num=a[i-1];	
+				for(int j=i-1;j<n;j++ )
+				{
+					if(a[j]-1==num)
+					{
+						int temp=a[i-1];
+						a[i-1]=a[i];
+						a[i]=temp;
+					}
+				}
+				break;
+			}
+		}
+		for(int i=0;i<n;i++)
+		{
+			System.out.println(a[i]);
+		}
+	}
+	public static void main(String [] args)
+	{
+		int a[]= {1,5,8,4,7,6,5,3,1};
+		permute(a);
+	}
+
+}
diff --git a/code/src/com/allendowney/thinkdast/NobelInt.java b/code/src/com/allendowney/thinkdast/NobelInt.java
new file mode 100644
index 00000000..d82680a1
--- /dev/null
+++ b/code/src/com/allendowney/thinkdast/NobelInt.java
@@ -0,0 +1,39 @@
+import java.util.*;
+public class NobelInt {
+	public static void findnobelInt(Vector<Integer>v)
+	{
+		int min=v.get(0);
+		int n=v.size();
+		for(int i=0;i<n;i++) {
+		if(v.get(i)<0)
+		{
+			min=0;
+		
+		}
+		if(min>v.get(i))
+		{
+			min=v.get(i);
+			
+		}
+		}
+		if(min==n-1)
+		{
+			System.out.println("Nobel Integer :"+ min);
+		}
+		else
+		{
+			System.out.println("Nobel Integer: "+0);
+		}
+	}
+public static void main(String []args)
+{
+	Vector <Integer>vec=new Vector<>();
+	vec.add(15);
+	vec.add(17);
+	vec.add(18);
+	vec.add(14);
+	vec.add(4);
+	findnobelInt(vec);
+	
+}
+}
diff --git a/code/src/com/allendowney/thinkdast/Parenthisischecker.java b/code/src/com/allendowney/thinkdast/Parenthisischecker.java
new file mode 100644
index 00000000..aacfc957
--- /dev/null
+++ b/code/src/com/allendowney/thinkdast/Parenthisischecker.java
@@ -0,0 +1,46 @@
+import java.util.*;
+public class Parenthisischecker {
+	static void checkparent(String str)
+	{
+	
+		Stack<Character>s=new Stack<>();
+		
+		
+		
+		for(int i1=0;i1<str.length();i1++)
+		{
+			char current=str.charAt(i1);
+			if(str.charAt(i1)=='['||str.charAt(i1)=='('||str.charAt(i1)=='{')
+			{
+				s.push(current);
+				
+			}
+			else if(str.charAt(i1)==']'||str.charAt(i1)==')'||str.charAt(i1)=='}')
+			{
+				if(s.isEmpty())
+				{
+					System.out.print("Not Balanced");
+				}
+				char last=s.peek();
+				if(current == '}' && last == '{' || current == ')' && last == '(' || current == ']' && last == '[')
+				{
+					s.pop();
+				}
+			}
+			
+		}
+		if(s.isEmpty())
+		{
+			System.out.print("Balanced ");
+		}
+		else
+		{
+			System.out.print("Not Balanced");
+		}
+	}
+public static void main(String [] args)
+{
+	String str="{}[][";
+	checkparent(str);
+}
+}