add code for insert digit anywhere

not as clean as the python version due to the data structure

java/insert_digit_anywhere/Main.java

@@ -0,0 +1,20 @@
+package insert_digit_anywhere;
+
+import java.util.ArrayList;
+
+public class Main
+{
+    public static void main(String[] args)
+    {
+        Util.init();
+
+        /* By changing the args[0] we can modify which prime we want to be the 
+         * first in the row. Same for the number of iteration.
+         */
+        Tree tree = new Tree(Integer.parseInt(args[0]), new ArrayList<Long>());
+        for (int i = 0; i < Integer.parseInt(args[1]); i++) {
+            tree.step();
+            Util.printList(tree.longestPath());
+        }
+    }
+}

java/insert_digit_anywhere/Tree.java

@@ -0,0 +1,84 @@
+package insert_digit_anywhere;
+
+import java.util.ArrayList;
+import java.util.Arrays;
+
+class Tree
+{
+    private long value;
+    private ArrayList<Tree> children;
+
+    public Tree(long tree_value, ArrayList<Long> tree_children)
+    {
+        value = tree_value;
+        children = new ArrayList<Tree>();
+        for (long child : tree_children) {
+            children.add(new Tree(child, new ArrayList<Long>()));
+        }
+    }
+
+    public void step()
+    {
+        if (children.isEmpty()) {
+            ArrayList<Long> xs = next(value);
+            for (long val : xs) {
+                children.add(new Tree(val, new ArrayList<Long>()));
+            }
+            return;
+        }
+        for (Tree child : children) {
+            child.step();
+        }
+    }
+
+    public ArrayList<Long> longestPath()
+    {
+        ArrayList<Long> retval = new ArrayList<Long>();
+        if (children.isEmpty()) {
+            retval.add(value);
+            return retval;
+        }
+        int max_length = 0;
+        for (Tree child : children) {
+            ArrayList<Long> temp = child.longestPath();
+            if (temp.size() > max_length) {
+                max_length = temp.size();
+                retval = temp;
+            }
+        }
+        retval.add(0, value);
+        return retval;
+    }
+
+    /* modify this function to change functionality */
+    public ArrayList<Long> next(long val)
+    {
+        /* Super messy but does work
+         * TODO cleaning/comment
+         */
+        ArrayList<Long> new_xs = new ArrayList<Long>();
+        String[] s =  Long.toString(val).split("");
+        for(int i = 0; i <= s.length; i++){
+            String[] temp = new String[s.length + 1];
+            int j;
+            for(j = 0; j < i; j++){
+                temp[j] = s[j];
+            }
+            for(int k = j + 1; k < temp.length; k++){
+                temp[k] = s[k - 1];
+            }
+            
+            for(int num = 0; num <= 9; num++){
+                temp[j] = String.valueOf(num);
+                String str = Arrays.toString(temp)
+                                   .replaceAll("[^\\d.]", "")
+                                   .replaceAll("^0+(?!$)", "");
+                Long test = Long.parseLong(str);
+                if(Util.isPrime(test) && test != val){
+                    new_xs.add(test);
+                }
+            }
+        }
+        return new_xs; 
+    }
+}

java/insert_digit_anywhere/Util.java

@@ -0,0 +1,69 @@
+package insert_digit_anywhere;
+
+import java.util.HashMap;
+import java.util.Iterator;
+import java.util.stream.IntStream;
+import java.util.ArrayList;
+
+class Util
+{
+    private static HashMap<Long, Boolean> primes;
+    private static HashMap<Long, Boolean> squareFree;
+
+    public static void init()
+    {
+        primes = new HashMap<Long, Boolean>();
+        squareFree = new HashMap<Long, Boolean>();
+    }
+
+    public static boolean isPrime(long val)
+    {
+        // Add 2 to the HashMap in advance in order to quickly exclude even number later
+        primes.put(2l, true);
+        if (primes.containsKey(val)) {
+            return primes.get(val);
+        }
+        if (val < 2) {
+            primes.put(val, false);
+            return false;
+        }
+        /* I tried to make this process more readable but I'm not sure about the efficiency 
+         * TODO check the efficiency of this new method
+         */
+        boolean temp = (val % 2) != 0 
+                        && 
+                        IntStream.rangeClosed(3, (int) Math.sqrt(val))
+                        .filter(n -> n % 2 != 0)
+                        .noneMatch(n -> (val % n == 0));
+        primes.put(val, temp);
+        return temp;
+    }
+
+    public static boolean isSquareFree(long val)
+    {
+        if (squareFree.containsKey(val)) {
+            return squareFree.get(val);
+        }
+        for (int i = 2; i * i <= val; i++) {
+            if (isPrime(i)) {
+                if (val % (i * i) == 0) {
+                    squareFree.put(val, false);
+                    return false;
+                }
+            }
+        }
+        squareFree.put(val, true);
+        return true;
+    }
+
+    // Did some cleaning to this util method
+    public static void printList(ArrayList<Long> ls)
+    {
+        Iterator<Long> it = ls.iterator();
+        System.out.printf("[%s",it.next());
+        do{
+            System.out.printf(", %s", it.next());
+        }while(it.hasNext());
+        System.out.print("]\n");
+    }
+}