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) {
        ListNode* res;
        unordered_map<ListNode*, int> hash_table;
        ListNode* pNode = pHead;
        while (true) {
            if (pNode == nullptr) {
                return nullptr;
            } else {
                if (hash_table.find(pNode) == hash_table.end()) { //没找到
                    hash_table[pNode] = 1;
                } else { //找到了
                    res = pNode;
                    break;
                }
                pNode = pNode->next;
            }

        }
        return res;
    }
};