Add Two Numbers II LeetCode solution in C++

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/**
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 * Definition for singly-linked list.
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 * struct ListNode {
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 *     int val;
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 *     ListNode *next;
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 *     ListNode() : val(0), next(nullptr) {}
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 *     ListNode(int x) : val(x), next(nullptr) {}
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 *     ListNode(int x, ListNode *next) : val(x), next(next) {}
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 * };
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 */
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class Solution {
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 public:
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  ListNode *reverse(ListNode *l1) {
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    if (l1 == NULL || l1->next == NULL) {
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      return l1;
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    }
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    ListNode *ans = reverse(l1->next);
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    l1->next->next = l1;
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    l1->next = NULL;
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    return ans;
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  }
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  ListNode *addTwoNumbers(ListNode *l1, ListNode *l2) {
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    ListNode *l11 = reverse(l1);
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    ListNode *l22 = reverse(l2);
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    ListNode *dummy = new ListNode(0);
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    ListNode *curr = dummy;
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    int carry = 0;
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    int sum = 0;
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    while (l11 || l22 || carry != 0) {
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      int x = l11 ? l11->val : 0;
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      int y = l22 ? l22->val : 0;
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      sum = x + y + carry;
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      carry = sum / 10;
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      curr->next = new ListNode(sum % 10);
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      curr = curr->next;
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      l11 = l11 ? l11->next : nullptr;
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      l22 = l22 ? l22->next : nullptr;
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    }
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    dummy = dummy->next;
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    dummy = reverse(dummy);
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    return dummy;
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  }
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};