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)
/*
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;
}
};