【Leetcode】142. Linked List Cycle II

142. Linked List Cycle II

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?

思路

首先证明链表有环，再让一个指针从头开始，另一个指针从相遇的位置，每次都只走一步，相遇的地方就是链表的环开始的地方。

代码

public ListNode detectCycle(ListNode head) {
    if (head == null) return null;
    boolean flag = false;
    ListNode fast = head, slow = head;

    while (fast != null && fast.next != null) {
        fast = fast.next.next;
        slow = slow.next;
        if (fast == slow) {
            flag = true;
            break;
        }
    }

    if (!flag) return null;

    slow = head;
    while (true) {
    	//先判断，避免只有两个节点的环的情况
        if (slow == fast) return slow;
        slow = slow.next;
        fast = fast.next;
    }
}