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) {
    }
};
*/
#include <asm-generic/errno.h>
class Solution {
  public:
    ListNode* EntryNodeOfLoop(ListNode* pHead) {
        struct ListNode* f_node = nullptr;
        struct ListNode* s_node = nullptr;

        if (pHead->next == nullptr && pHead->next->next == nullptr)
            return nullptr;
        f_node = pHead;
        s_node = pHead;
        while (s_node->next != nullptr && f_node->next->next != nullptr) {
            s_node = s_node->next;
            f_node = f_node->next->next;
            if (f_node == s_node) {
                f_node = pHead;
                while (f_node->next != nullptr && s_node->next != nullptr) {
                    if (f_node == s_node)
                        return f_node;
                    f_node = f_node->next;
                    s_node = s_node->next;
                }
                return nullptr;
            }
        }
        return nullptr;

    }
};