enh(JavaScript): add getByIndex and insertAfterByIndex on singly linked list (#516)

algorithms/JavaScript/src/linked-lists/singly.js

@@ -10,6 +10,39 @@ class SinglyLinkedList {
     this.head = null;
   }
 
+  // Will return the element at that particular index in linklist starting
+  // from head.
+  getByIndex(idx) {
+    if (!this.head) {
+      return 'List is Empty';
+    }
+
+    let currentNode = this.head;
+    let iterationCount = 0;
+
+    for (
+      currentNode = this.head;
+      currentNode != null;
+      currentNode = currentNode.next
+    ) {
+      if (iterationCount == idx) {
+        return currentNode;
+      }
+      iterationCount++;
+    }
+
+    return null;
+  }
+
+  setByIndex(idx, newValue) {
+    const nodeElem = this.getByIndex(idx);
+    if (nodeElem) {
+      nodeElem.data = newValue;
+      return nodeElem;
+    }
+
+    return null;
+  }
   insertAtHead(data) {
     const newNode = new Node(data);
     if (this.head == null) {
@@ -20,6 +53,47 @@ class SinglyLinkedList {
     }
   }
 
+  // idx : index of the node after which you want to
+  // insert the new node;
+  // data: value associated with new node.
+  insertAfterByIndex(idx, data) {
+    let afterNode;
+    let newNode;
+
+    const beforeNode = this.getByIndex(idx);
+
+    if (beforeNode) {
+      afterNode = beforeNode.next;
+      newNode = new Node(data);
+
+      beforeNode.next = newNode;
+      newNode.next = afterNode;
+
+      return this;
+    }
+
+    return null;
+  }
+
+  // idx: index of the node which you want to remove from linklist.
+  removeFromMiddle(idx) {
+    const beforeNode = this.getByIndex(idx - 1);
+    let afterNode;
+    let nodeToRemove;
+
+    if (beforeNode) {
+      nodeToRemove = beforeNode.next;
+
+      if (nodeToRemove) {
+        afterNode = nodeToRemove.next;
+        beforeNode.next = afterNode;
+
+        return this;
+      }
+    }
+    return null;
+  }
+
   removeAtHead() {
     // check for underflow
     if (this.head == null) {
@@ -50,6 +124,34 @@ class SinglyLinkedList {
     return this;
   }
 
+  // Removes elements which are at the end of the linklist.
+  removeFromEnd() {
+    if (!this.head) {
+      console.log('List is Empty');
+      return this;
+    }
+
+    let returnNode;
+    if (!this.head.next) {
+      returnNode = this.head;
+      this.head = null;
+      return returnNode;
+    }
+
+    let currentNode = this.head.next;
+    let previousNode = this.head;
+
+    while (currentNode) {
+      if (currentNode.next) {
+        previousNode = currentNode;
+      }
+      currentNode = currentNode.next;
+    }
+    returnNode = previousNode.next;
+    previousNode.next = null;
+    return returnNode;
+  }
+
   printList() {
     // if head is null then list is empty
     if (this.head == null) {
@@ -71,5 +173,15 @@ array.insertAtHead('xyz');
 array.insertAtHead(1.1);
 array.removeAtHead();
 array.insertAtEnd(99);
+array.insertAtEnd(100);
+array.insertAtEnd(101);
+array.insertAtEnd(102);
+array.insertAtEnd(103);
+array.insertAtEnd(104);
+array.insertAtEnd(105);
+
 array.printList();
 
+array.removeFromEnd();
+
+array.printList();