Linked List Cycle II LeetCode solution in C++

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class Solution {
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 public:
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  ListNode *detectCycle(ListNode *head) {
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    map<ListNode *, int> m;
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    int index = 0;
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    while (head) {
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      if (m.find(head) == m.end())
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        m[head] = index;
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      else
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        return (head);
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      head = head->next;
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      index++;
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    }
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    return (NULL);
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  }
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};