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# Recursive function to add 5 to alternate nodes of the linked list

- October 25, 2016
- Posted by: Kamal Rawat
- Category: Algorithms

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Write a recursive function that add 5 to the alternate nodes of the linked list. For example, if the list is

1 -> 2 -> 3 -> 4 -> 5 -> 6 -> 7 -> 8 -> 9 -> 10

then the function should change the list to the below one

1 -> 7 -> 3 -> 9 -> 5 -> 11 -> 7 -> 13 -> 9 -> 15

**Iterative Solution:**

The code to add 5 to alternate nodes is straight forward. We will just skip the alternate nodes. Below is the iterative solution for the same:

void addToAlternate(Node* h) { while(h != NULL) { h = h->next; // Now we are at the node where we need to add 5 if(h != NULL) { h->data += 5; h = h->next; } } }

**Recursive Solution:**

In recursion we can use a flag that indicate if we need to skip the current node or add 5 to it

//If node need to be skipped then skipNode=1, else 0 void addToAlternate(Node* h, int skipNode = 1) { if(h == NULL) return; if(skipNode) { // Do Nothing. Just call the recursive function addToAlternate(h->next,0); } else { // Add 5 to the current node h->data += 5; addToAlternate(h->next, 1); } }

The above code can be more clearly written as below

//If node need to be skipped then skipNode=1, else 0 void addToAlternate(Node* h, int skipNode = 1) { if(h == NULL) return; if(!skipNode) { // Add 5 to the current node h->data += 5; } addToAlternate(h->next, !skipNode); }

Another way can be to look at pair of the node,

- skip the first Node,
- Add 5 to the second Node
- Call the function recursively for the third Node

void addToAlternate(Node* h) { if(h == NULL || h->next == NULL) return; // Skip the first node h = h->next; // Add 5 to the second node h->data += 5; // Call recursively for the 3rd Node addToAlternate(h->next); }

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