剑指Offer——JZ55.链表环的入口结点【快慢指针】

题目传送门

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题解

• **解法一：**记录走过的结点，遇到走过的判定为有环
• 解法二：
  

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AC-Code

/*
struct ListNode {
    int val;
    struct ListNode *next;
    ListNode(int x) :
        val(x), next(NULL) {
    }
};
*/
class Solution {
     
public:
    ListNode* EntryNodeOfLoop(ListNode* pHead) {
     
        set<ListNode*> st;
        while(pHead) {
     
            if(st.count(pHead)) {
     
                return pHead;
            }
            st.insert(pHead);
            pHead = pHead->next;
        }
        return NULL;
    }
};

class Solution {
     
public:
    ListNode* EntryNodeOfLoop(ListNode* pHead) {
     
        ListNode *fast = pHead;
        ListNode *slow = pHead;
        while (fast && fast->next) {
     
            fast = fast->next->next;
            slow = slow->next;
            if (fast == slow) break;
        }
        if (!fast || !fast->next) return nullptr;
        fast = pHead;
        while (fast != slow) {
     
            fast = fast->next;
            slow = slow->next;
        }
        return fast;
    }
};