题解 | 链表中环的入口结点

/*
struct ListNode {
    int val;
    struct ListNode *next;
    ListNode(int x) :
        val(x), next(NULL) {
    }
};
*/
class Solution {
public:
    ListNode* EntryNodeOfLoop(ListNode* pHead) {
        if (pHead == nullptr) return nullptr;
        ListNode* fastNode = pHead;
        ListNode* slowNode = pHead;
        while (fastNode&&fastNode->next) {
            fastNode = fastNode->next->next;
            slowNode = slowNode->next;
            if(fastNode == slowNode) {
                while (slowNode!=pHead) {
                    pHead = pHead->next;
                    slowNode = slowNode->next;
                }
                return slowNode;
            }
        }
        return nullptr;
    }
};