Formatted question description: https://leetcode.ca/all/2181.html

2181. Merge Nodes in Between Zeros (Medium)

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.

Similar Questions:

Solution 1.

// OJ: https://leetcode.com/problems/merge-nodes-in-between-zeros/

// Time: O(N)
// Space: O(1)
class Solution {
public:
    ListNode* mergeNodes(ListNode* head) {
        ListNode dummy, *tail = &dummy;
        while (head) {
            if (head->val == 0) head = head->next; // skip leading `0`
            if (!head) break;
            int sum = 0;
            while (head->val != 0) { // sum numbers before the next `0`
                sum += head->val;
                head = head->next;
            }
            tail->next = new ListNode(sum); // append `sum`
            tail = tail->next;
        }
        return dummy.next;
    }
};

Solution 2.

If we are asked to do it in-place.

// OJ: https://leetcode.com/problems/merge-nodes-in-between-zeros/

// Time: O(N)
// Space: O(1) as it's done in-place
class Solution {
public:
    ListNode* mergeNodes(ListNode* head) {
        ListNode dummy, *tail = &dummy;
        while (head) {
            if (head->val == 0) head = head->next;
            if (!head) break;
            auto node = head;
            head = head->next;
            while (head->val != 0) {
                node->val += head->val;
                head = head->next;
            }
            tail->next = node;
            tail = tail->next;
            node->next = nullptr;
        }
        return dummy.next;
    }
};

If you are asked to add the code to free node:

// OJ: https://leetcode.com/problems/merge-nodes-in-between-zeros/

// Time: O(N)
// Space: O(1) as it's done in-place
class Solution {
public:
    ListNode* mergeNodes(ListNode* head) {
        ListNode dummy, *tail = &dummy;
        while (head) {
            if (head->val == 0) {
                auto p = head;
                head = head->next;
                delete p;
            }
            if (!head) break;
            auto node = head;
            head = head->next;
            while (head->val != 0) {
                node->val += head->val;
                auto p = head;
                head = head->next;
                delete p;
            }
            tail->next = node;
            tail = tail->next;
            node->next = nullptr;
        }
        return dummy.next;
    }
};

Discuss

https://leetcode.com/problems/merge-nodes-in-between-zeros/discuss/1784739/

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