<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) {}
 * };
 */
#pragma GCC optimize("O3,unroll-loops")
#pragma GCC target("avx2,bmi,bmi2,lzcnt,popcnt")

static const bool Booster = [](){
    std::ios_base::sync_with_stdio(false);
    std::cin.tie(nullptr);
    std::cout.tie(nullptr);
    return true;
}();

inline bool is_digit(char c) {
    return c >= '0' && c <= '9';
}
class Solution {
public:
    ListNode* mergeNodes(ListNode* head) {
        if(!head) return head;
        ListNode* slow = head;
        ListNode* fast = slow->next;
        ListNode* lastNode = head;
        int sum = 0;
        while(fast){
            if(fast->val!=0){
                sum += fast->val;
            }
            else{
                slow->val = sum;
                sum = 0;
                lastNode = slow;
                slow = slow->next;
            }
            fast = fast->next;
        }
        lastNode->next = nullptr;
        while(slow){
            ListNode* nxt = slow->next;
            delete slow;
            slow = nxt;
        }
        return head;
    }
};
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