leetcode 日经贴，Cpp code -Linked List Cycle II

/**
* Definition for singly-linked list.
* struct ListNode {
*     int val;
*     ListNode *next;
*     ListNode(int x) : val(x), next(NULL) {}
* };
*/
class Solution {
public:
ListNode *detectCycle(ListNode *head) {
if (!head || !head->next) {
return NULL;
}
ListNode *p1 = head, *p2 = head->next;
while (p1 != p2 && p2) {
p1 = p1->next;
if (p2->next) {
p2 = p2->next->next;
} else {
p2 = p2->next;
}
}
if (p1 != p2) {
return NULL;
}
p1 = head;
p2 = p2->next;
while (p1 != p2) {
p1 = p1->next;
p2 = p2->next;
}
return p1;
}
};