题解 | #链表中环的入口结点#

快慢指针应用

/*
struct ListNode {
    int val;
    struct ListNode *next;
    ListNode(int x) :
        val(x), next(NULL) {
    }
};
*/
class Solution {
  public:
    ListNode* EntryNodeOfLoop(ListNode* pHead) {
        ListNode* slow = pHead, *fast = pHead;
        while (fast != NULL && fast->next != NULL) {
            slow = slow->next;
            fast = fast->next->next;
            if (fast == slow) {
                break;
            }
        }
        if (fast == NULL || fast->next == NULL) {
            return NULL;
        }
        ListNode* p1 = pHead, *p2 = slow;
        while (p1 != p2) {
            p1 = p1->next;
            p2 = p2->next;
        }
        return p1;
    }
};