2181. Merge Nodes in Between Zeros

2181. Merge Nodes in Between Zeros

Difficulty: Medium

Topics: Linked List, Simulation

You are given the head of a linked list, which contains a series of integers separated by 0's. The beginning and end of the linked list will have Node.val == 0.

For every two consecutive 0's, merge all the nodes lying in between them into a single node whose value is the sum of all the merged nodes. The modified list should not contain any 0's.

Return the head of the modified linked list.

Example 1:

• Input: head = [0,3,1,0,4,5,2,0]
• Output: [4,11]
• Explanation: The above figure represents the given linked list. The modified list contains
  • The sum of the nodes marked in green: 3 + 1 = 4.
  • The sum of the nodes marked in red: 4 + 5 + 2 = 11.

Example 2:

• Input: head = [0,1,0,3,0,2,2,0]
• Output: [1,3,4]
• Explanation: The above figure represents the given linked list. The modified list contains
  • The sum of the nodes marked in green: 1 = 1.
  • The sum of the nodes marked in red: 3 = 3.
  • The sum of the nodes marked in yellow: 2 + 2 = 4.

Constraints:

• The number of nodes in the list is in the range [3, 2 * 105].
• 0 <= Node.val <= 1000
• There are no two consecutive nodes with Node.val == 0.
• The beginning and end of the linked list have Node.val == 0.

Hint:

1. How can you use two pointers to modify the original list into the new list?
2. Have a pointer traverse the entire linked list, while another pointer looks at a node that is currently being modified.
3. Keep on summing the values of the nodes between the traversal pointer and the modifying pointer until the former comes across a ‘0’. In that case, the modifying pointer is incremented to modify the next node.
4. Do not forget to have the next pointer of the final node of the modified list point to null.

Solution:

We can follow these steps:

1. Parse Input Linked List: Read the input linked list and identify segments between zeros.
2. Sum Segments: For each segment between two zeros, calculate the sum of the nodes.
3. Create New Linked List: Create a new linked list containing the sums of the segments, with zeros at the start and end.

Let's implement this solution in PHP: 2181. Merge Nodes in Between Zeros

<?php
/**
 * @param ListNode $head
 * @return ListNode
 */
function mergeNodes($head) {
    ...
    ...
    ...
    /**
     * go to ./solution.php
     */
}

// Helper function to print linked list
function printLinkedList($head) {
    $current = $head;
    while ($current !== null) {
        echo $current->val . " ";
        $current = $current->next;
    }
    echo "\n";
}

// Helper function to create linked list from array
function createLinkedList($arr) {
    $dummy = new ListNode(0);
    $current = $dummy;
    foreach ($arr as $value) {
        $current->next = new ListNode($value);
        $current = $current->next;
    }
    return $dummy->next;
}

// Example usage
$input = [0, 3, 1, 0, 4, 5, 2, 0];
$head = createLinkedList($input);
$result = mergeNodes($head);
printLinkedList($result); // Expected output: 4 11
?>

Explanation:

1. Initialization:

   • We use a dummy node to facilitate list operations. This helps us easily return the new list starting from dummy->next.
   • sum is initialized to 0 to accumulate values between zeros.

2. Processing the List:

   • We skip the first node since it's a zero.
   • We iterate through the list, summing values until we encounter a zero.
   • When a zero is encountered, we create a new node with the current sum and append it to the new list. Then, we reset the sum to 0.

3. Creating and Printing Linked Lists:

   • The helper functions createLinkedList and printLinkedList are used to build a linked list from an array and print the linked list, respectively.

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