<p>You are given the <code>head</code> of a linked list, which contains a series of integers <strong>separated</strong> by <code>0</code>&#39;s. The <strong>beginning</strong> and <strong>end</strong> of the linked list will have <code>Node.val == 0</code>.</p>

<p>For <strong>every </strong>two consecutive <code>0</code>&#39;s, <strong>merge</strong> all the nodes lying in between them into a single node whose value is the <strong>sum</strong> of all the merged nodes. The modified list should not contain any <code>0</code>&#39;s.</p>

<p>Return <em>the</em> <code>head</code> <em>of the modified linked list</em>.</p>

<p>&nbsp;</p>
<p><strong class="example">Example 1:</strong></p>
<img alt="" src="https://assets.leetcode.com/uploads/2022/02/02/ex1-1.png" style="width: 600px; height: 41px;" />
<pre>
<strong>Input:</strong> head = [0,3,1,0,4,5,2,0]
<strong>Output:</strong> [4,11]
<strong>Explanation:</strong> 
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.
</pre>

<p><strong class="example">Example 2:</strong></p>
<img alt="" src="https://assets.leetcode.com/uploads/2022/02/02/ex2-1.png" style="width: 600px; height: 41px;" />
<pre>
<strong>Input:</strong> head = [0,1,0,3,0,2,2,0]
<strong>Output:</strong> [1,3,4]
<strong>Explanation:</strong> 
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.
</pre>

<p>&nbsp;</p>
<p><strong>Constraints:</strong></p>

<ul>
	<li>The number of nodes in the list is in the range <code>[3, 2 * 10<sup>5</sup>]</code>.</li>
	<li><code>0 &lt;= Node.val &lt;= 1000</code></li>
	<li>There are <strong>no</strong> two consecutive nodes with <code>Node.val == 0</code>.</li>
	<li>The <strong>beginning</strong> and <strong>end</strong> of the linked list have <code>Node.val == 0</code>.</li>
</ul>

/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     ListNode *next;
 *     ListNode() : val(0), next(nullptr) {}
 *     ListNode(int x) : val(x), next(nullptr) {}
 *     ListNode(int x, ListNode *next) : val(x), next(next) {}
 * };
 */
class Solution {
public:
    void insertAtTail(ListNode* &head, ListNode* &tail, int val){
        if(!head){
            ListNode* newNode = new ListNode(val);
            head = newNode;
            tail = newNode;
        }
        else{
            ListNode* newNode = new ListNode(val);
            tail->next = newNode;
            tail = newNode;
        }
    }
    ListNode* mergeNodes(ListNode* head) {
        // ListNode* head2 = NULL;
        // ListNode* tail2 = NULL;
        // ListNode* temp = head;
        // int s = 0;

        // while(temp){
        //     if(temp->val == 0 && s != 0){
        //         insertAtTail(head2, tail2, s);
        //         s = 0;
        //     }
        //     if(temp->val != 0){
        //         s += temp->val;
        //     }
        //     temp = temp->next;
        // }
        
        // return head2;

        //method 2
        ListNode* prev = NULL;
        ListNode* curr = head;
        ListNode* head2 = NULL;
        ListNode* temphead = head;
        int s = 0;

        while(curr){
            if(curr->val == 0 && prev){
                
                ListNode* nextNode = temphead->next;
                prev->next = NULL;
                temphead->next = curr;
                delete nextNode;
                curr->val = s;
                s = 0;
                if(temphead->val == 0) head2 = curr;
                temphead = curr;
                prev = curr;
                curr = curr->next;
            }
            else{
                s += curr->val;
                prev = curr;
                curr = curr->next;
            }
        }
        return head2;
    }
};
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