Recursion in C - Explain Given Solution

Question

Question: Write a recursive function to add to each number in a linked list the original values of its two neighbors. For example, 10->2->3->5->NULL becomes 12->15->10->8->NULL. Each node in the list has an integer number (int number) and a pointer to the next node (NODE

*next). Assume the list is not empty.

The answer given is below. I don't understand how to get this answer. Could someone explain?

int add (NODE *p, int a) {

if (p == NULL)

return 0;

int n, r;

r = add(p->next, p->number);

n = p->number;

p->number += a+r;

return n;

Answer 1

The `add` function is a recursive function that takes two arguments: a pointer to the current node (`NODE *p`) and an integer `a`, which is the number from the node before the current node.

Let's break down what happens during the execution of this function:

1. First, it checks if the current node is `NULL`. If it is, the function returns `0`. This serves as the base case of our recursion and it also accounts for the end of the list, when there's no more nodes to process.

2. If the current node is not `NULL`, it then recursively calls `add(p->next, p->number)`. This recursive call moves to the next node in the list and passes the number from the current node as the second argument. The result of this recursive call is stored in `r`.

3. The number from the current node is then stored in `n`. This is done after the recursive call to ensure that we're storing the original value of the node, before it gets changed.

4. The number in the current node is then updated to be the sum of its original value, `a` (the number from the previous node), and `r` (the result from the recursive call, which represents the original value of the next node).

5. Finally, it returns `n`, the original value of the current node. This will be used as `a` in the next recursion level, when processing the previous node.

The result of this function is a modified linked list, where each node's number is the sum of its original value and the original values of its two neighbors.

The tricky part of understanding this function is realizing that due to recursion, the processing order of nodes is reversed. It starts from the end of the list, moving towards the start, so the "next" node for any given node is actually processed before the current one. This ensures that we're adding the original values of the neighbors to each node, not their modified versions.