[Leetcode Week6]Linked List Cycle II
Linked List Cycle II 题解
题目来源:https://leetcode.com/problems/linked-list-cycle-ii/description/
Description
Given a linked list, return the node where the cycle begins. If there is no cycle, return null
.
Note: Do not modify the linked list.
Follow up:
Can you solve it without using extra space?
Solution
class Solution {
public:
ListNode *detectCycle(ListNode *head) {
if (head == NULL)
return NULL;
ListNode *slow = head, *fast = head;
slow = slow -> next;
fast = fast -> next;
if (fast == NULL || fast -> next == NULL)
return NULL;
else
fast = fast -> next;
while (slow != fast && slow != NULL && fast != NULL) {
slow = slow -> next;
fast = fast -> next;
if (fast == NULL || fast -> next == NULL)
return NULL;
else
fast = fast -> next;
}
if (slow != fast)
return NULL;
slow = head;
while (slow != fast) {
slow = slow -> next;
fast = fast -> next;
}
return slow;
}
};
解题描述
这道题同样是带环链表系列的题目,是找到链表环的起点。在判断链表是否有环的基础上要增加对环的起点的判断,这就需要搞清楚其中的数学关系:
设链表环起点距离链表头StartLen,链表环的长度为CycleLen,快慢游标第一次相遇的位置距离链表环起点长度为d(不必求出),则有
对慢游标,s1 = StartLen + n * CycleLen + d,
对快游标,s2 = StartLen + m * CycleLen + d
而s2 = 2 * s1
则有 StartLen = (m - 2 * n) * CycleLen - d
所以当第一次相遇之后,将慢游标设为head,然后快慢游标每次只向后移动一个节点,则再次相遇的位置会在链表环的起点
具体参考博客:判断单向链表是否有环,环起点,环长,链表长
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