题解 | 链表中环的入口结点

/*
struct ListNode {
    int val;
    struct ListNode *next;
    ListNode(int x) :
        val(x), next(NULL) {
    }
};
*/
class Solution {
  public:
    ListNode* EntryNodeOfLoop(ListNode* pHead) {
        if(pHead == nullptr)
            return nullptr;
        ListNode *fast = pHead->next;
        ListNode *low = pHead;
       //快指针走2步，慢指针走一步
        if(fast != nullptr){
            low = low->next;
            fast = fast->next;
        }
        else return nullptr;
        
        while(fast != low){
            if(fast == nullptr || fast->next == nullptr)
                return nullptr;
            fast = fast->next->next;
            low = low->next;
        }
        printf("fast:%d  low: %d\n",fast->val,low->val);
        if(fast == low){
           
            fast = pHead;
            while(fast != low)
            {           
                fast = fast->next;
                low = low->next;
            } 
            return fast;
        }
        else return nullptr;
    }
};