LeetCode Linked List Cycle II

题意：给出一个链表，判断是否有环，如果有环，输出环的起点，如果没有，输出null

思路：从起点开始，一个每次走一步，一个每次走两步，如果两点出现重合，说明有环，此时，一个从起点开始，另一个从重合点开始，两个每次走一次，此时重合点就是环的起点

代码如下：

class Solution {public ListNode detectCycle(ListNode head) {ListNode slow = head;if (null == head) return null;ListNode fast = head;do {if (null == fast || null == slow) return null;slow = slow.next;if (null == fast.next) return null;fast = fast.next.next;}while (fast != slow);slow = head;while (fast != slow){fast = fast.next;slow = slow.next;}return slow;}
}

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