LeetCode 142. 环形链表 II

思路：
快慢指针法：当快指针与慢指针相遇时，分别从起点，相遇点开始走，相遇即为环入口

/*** 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) {ListNode* slow = head;ListNode* fast = head;while (fast && fast ->next) {slow = slow ->next;fast = fast ->next ->next;if (slow == fast) {ListNode* index1 = slow;ListNode* index2 = head;while (index1 != index2) {index1 = index1 ->next;index2 = index2 ->next;}return index2;}}return NULL;}
};