先在链表头分别设一个快指针和慢指针,快指针一次走两步,慢指针一次走一步,一直到相遇。 接下来在链表头和相遇点分别设置两个慢指针a和b,当a再次走到上面的相遇点时,b走到环的起点。(证明见下图) alt

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