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Recursion for Coding Interviews in Java

Sum Numbers in a Linked List

Explore how to sum all the values in a linked list using recursion in Java. Understand the base and recursive cases, see working code examples, and learn to traverse a list while summing nodes efficiently.

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Sum n numbers

When given a linked list, sum the values of all the nodes and then return the sum.

The following illustration explains this concept:

Implementing the code

The following code calculates the sum of the node values in the linked list using recursion.

Experiment with the code by changing the elements of the list to see how it works.

Java
class LinkedList {
    // Linked List Node
    static class Node {
      int value;
      Node next;
    };
    static int sumList(Node head) {
      // Base case
      if (head == null) {
        return 0;
      }
      // Recursive case
      else {
        return (head.value + (sumList(head.next)));
      }
    } 
    static Node insertAtHead(Node temp_head, int new_value) {
      Node new_Node = new Node();
      new_Node.value = new_value;
      new_Node.next = (temp_head);
      (temp_head) = new_Node;
      return temp_head;
    }
    public static void main( String args[] ) {
        // Empty Linked List
        Node head = null;
        // Linked List = 3->1->5->8->2
        head = insertAtHead(head, 2);        
        head = insertAtHead(head, 8);        
        head = insertAtHead(head, 5);        
        head = insertAtHead(head, 1);        
        head = insertAtHead(head, 3);
        System.out.println("Linked List: ");
        for (Node i = head; i != null; i = i.next) {
          System.out.print(i.value + " ");
        } 
        int sum = sumList(head);
        System.out.println(" ");
        System.out.println("Sum: " + sum);
    }
}

Understanding the code

The code given above can be broken down into two parts: ...