LC-141andLC-142

142. 环形链表 II

思路：

设链表共有 a+b 个节点，其中 链表头部到链表入口 有 a 个节点（不计链表入口节点）， 链表环 有 b 个节点。

再设两指针分别走了 f，s 步，则有：

• f = 2sf=2s；
• fast 比 slow多走了 n 个环的长度，即 f = s + nbf=s+nb；
• 以上两式相减得：f = 2nbf=2nb，s = nbs=nb；

概括一下：

根据：

1. f=2s （快指针每次2步，路程刚好2倍）
2. f = s + nb (相遇时，刚好多走了n圈）

推出：s = nb

从head结点走到入环点需要走 ： a + nb， 而slow已经走了nb，那么slow再走a步就是入环点了。

如何知道slow刚好走了a步？ 从head开始，和slow指针一起走，相遇时刚好就是a步。

Java实现

/**
 * Definition for singly-linked list.
 * class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode(int x) {
 *         val = x;
 *         next = null;
 *     }
 * }
 */
public class Solution {
    public ListNode detectCycle(ListNode head) {
        ListNode fast = head, slow = head;
        while(true) {
            if(fast == null || fast.next == null) return null;
            fast = fast.next.next;
            slow = slow.next;
            if(fast == slow) break;
        }
        fast = head;
        while (fast != slow){
            fast = fast.next;
            slow = slow.next;
        }
        return fast;
    }
}

Python实现

# Definition for singly-linked list.
# class ListNode:
#     def __init__(self, x):
#         self.val = x
#         self.next = None

class Solution(object):
    def detectCycle(self, head):
        fast, slow = head, head
        while True:
            if not (fast and fast.next): return
            fast, slow = fast.next.next, slow.next
            if fast == slow: break
        fast = head
        while fast != slow:
            fast, slow = fast.next, slow.next
        return fast

141. 环形链表

142少去算法部分。单纯用快慢指针判断是否相遇即刻。

Java实现

/**
 * Definition for singly-linked list.
 * class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode(int x) {
 *         val = x;
 *         next = null;
 *     }
 * }
 */
public class Solution {
    public boolean hasCycle(ListNode head) {
        ListNode fast = head, slow = head;
        while(true) {
            if(fast == null || fast.next == null) return false;
            fast = fast.next.next;
            slow = slow.next;
            if(fast == slow) return true;
        }
    }
}

Python实现

# Definition for singly-linked list.
# class ListNode:
#     def __init__(self, x):
#         self.val = x
#         self.next = None

class Solution:
    def hasCycle(self, head: Optional[ListNode]) -> bool:
        fast, slow = head, head
        while True:
            if not (fast and fast.next): return False
            fast = fast.next.next
            slow = slow.next
            if fast == slow: return True