Leetcode142. 环形链表 II

题目

142. 环形链表 II

代码（首刷看解析）

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

代码（7.30 二刷自解）

class Solution {
public:
    ListNode *detectCycle(ListNode *head) {
        auto fast = head;
        auto slow = head;
        if(!fast || !fast->next)
            return nullptr;
        do {
            if(!fast || !fast->next)
                return nullptr;
            fast = fast->next->next;
            slow = slow->next;
        }while(fast != slow);
        slow = head;
        while(slow != fast) {
            slow = slow->next;
            fast = fast->next;
        }
        return slow;
    }
};

代码（8.8 三刷自解）

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

GO

func detectCycle(head *ListNode) *ListNode {
    fast, slow := head, head
    for fast != nil && fast.Next != nil {
        fast = fast.Next.Next
        slow = slow.Next
        if fast == slow {
            break
        }
    }
    if fast == nil || fast.Next == nil {
        return nil
    }
    slow = head
    for fast != slow {
        slow = slow.Next
        fast = fast.Next
    }
    return fast
}