chore(Java): add fold-linked-list (#266)

algorithms/Java/README.md

@@ -18,6 +18,7 @@
 3. [Doubly](linked-lists/doubly.java)
 4. [Reverse](linked-lists/reverse.java)
 5. [Singly](linked-lists/singly.java)
+6. [Fold Linked List](linked-lists/fold-linked-list.java)
 
 ## Queues
 

algorithms/Java/linked-lists/fold-linked-list.java

@@ -0,0 +1,92 @@
+//    Java implementation for folding a linked list
+public static class Node {
+    int data;
+    Node next;
+    public Node(int data){
+        this.data = data;
+        this.next = null;
+    }
+}
+
+public static class LinkedList {
+    Node head;
+    Node tail;
+    int size;
+
+    public void display() {
+        for (Node temp = head; temp != null; temp = temp.next) {
+            System.out.print(temp.data + " ");
+        }
+        System.out.println();
+    }
+
+    public void addFirst(int val) {
+        Node temp = new Node(val);
+        temp.next = head;
+        head = temp;
+        if (size == 0) {
+            tail = temp;
+        }
+        size++;
+    }
+
+    void addLast(int val) {
+        Node temp = new Node(val);
+        temp.next = null;
+        if (size == 0) {
+            head = tail = temp;
+        } else {
+            tail.next = temp;
+            tail = temp;
+        }
+        size++;
+    }
+
+    Node fleft;
+    private void foldhelper(Node right , int floor){
+        if(right == null){
+            return;
+        }
+        foldhelper(right.next , floor+1);
+        if(floor>size/2){
+            Node val = fleft.next;
+            fleft.next = right;
+            right.next = val;
+            fleft = val;
+        }
+        else if(floor==size/2){
+            this.tail = right;
+            tail.next = null;
+        }
+        else{
+            // do nothing
+        }
+    }
+        
+    public void fold() {
+        fleft = this.head;
+        foldhelper(this.head, 0);
+    }
+}
+
+//Driver Code
+public static void main(String[] args) throws Exception {
+    LinkedList ll = new LinkedList();
+    int n = 6;
+    for (int i = 1; i <= n; i++) {
+        ll.addLast(i);
+    }
+    // LinkedList --> [1,2,3,4,5,6] for n = 6
+
+    System.out.print("Linked List before folding --> "); 
+    ll.display();
+    ll.fold();
+    System.out.print("Linked List after folding --> "); 
+    ll.display();
+}
+
+/*
+    Output:-
+    Linked List before folding --> 1 2 3 4 5 6 
+    Linked List after folding --> 1 6 2 5 3 4
+*/