142-linked-list-cycle-ii.cpp

class Solution {
public:
    
         ListNode *detectCycle(ListNode *head) {
    if (head == NULL || head->next == NULL) return NULL;
    
    ListNode* firstp = head;
    ListNode* secondp = head;
    bool isCycle = false;
    
    while(firstp != NULL && secondp != NULL) {
        firstp = firstp->next;
        if (secondp->next == NULL) return NULL;
        secondp = secondp->next->next;
        if (firstp == secondp) { isCycle = true; break; }
    }
    
    if(!isCycle) return NULL;
    firstp = head;
    while( firstp != secondp) {
        firstp = firstp->next;
        secondp = secondp->next;
    }

    return firstp;
}
    
};