LeetCode 142. Linked List Cycle II

Given a linked list, return the node where the cycle begins. If there is no cycle, return null.

Note: Do not modify the linked list.

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    ListNode *detectCycle(ListNode *head) {
        if(!head) return NULL;
        ListNode* slow = head;
        ListNode* fast = head;
        while(fast && fast->next && fast->next->next) {
            slow = slow->next;
            fast = fast->next->next;
            if(slow == fast) {
                break;
            }
        }
        if(!fast || !fast->next || !fast->next->next) return NULL; // remember to check the no circle condition.
        slow = head;
        while(slow != fast) {
            slow = slow->next;
            fast = fast->next;
        }
        return slow;
    }