142.linked-list-cycle-ii.cpp

/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     ListNode *next;
 *     ListNode(int x) : val(x), next(NULL) {}
 * };
 */
// 注意两种写法, fast为空或fast->next为空时无环
class Solution {
public:
    ListNode *detectCycle(ListNode *head) {
        if (!head) return nullptr;
        ListNode *slow = head, *fast = head;
        while (fast->next && fast->next->next) {
            slow = slow->next;
            fast = fast->next->next;
            if (slow == fast) break; // 加上这个条件, 一开始时slow是等于fast的
        }
        if (!fast->next || !fast->next->next) return nullptr; // 假如链表只有一个节点
        fast = head; // 这里赋值head
        while (slow != fast) {
            slow = slow->next;
            fast = fast->next;
        }
        return slow;
    }
};


class Solution {
public:
  ListNode *detectCycle(ListNode *head) {
    ListNode *slow = head;
    ListNode *fast = 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 fast;
  }
};