题解 | #链表中环的入口结点# C++ 解法，链表环入口

题解 | #链表中环的入口结点#
C++ 解法，链表环入口

/*
struct ListNode {
    int val;
    struct ListNode *next;
    ListNode(int x) :
        val(x), next(NULL) {
    }
};
*/
class Solution {
public:
    ListNode* EntryNodeOfLoop(ListNode* pHead) {
        if (pHead == nullptr || pHead->next == nullptr) return nullptr;
        ListNode *slow = pHead, *fast = slow;
        while (fast) {
            fast = fast->next;
            if (fast) {
                slow = slow->next;
                fast = fast->next;
            }
            if (slow == fast) { // 若进入，则有环
                fast = pHead;
                while (slow != fast) {
                    slow = slow->next;
                    fast = fast->next;
                }
                return slow;
            }
        }
        return nullptr;
    }
};