141/142-环形链表

LeetCode 141. Linked List Cycle

LeetCode-141

Given head, the head of a linked list, determine if the linked list has a cycle in it.

There is a cycle in a linked list if there is some node in the list that can be reached again by continuously following the next pointer. Internally, pos is used to denote the index of the node that tail’s next pointer is connected to. Note that pos is not passed as a parameter.

Return true if there is a cycle in the linked list. Otherwise, return false.

Example 1:

Input: head = [3,2,0,-4], pos = 1
Output: true
Explanation: There is a cycle in the linked list, where the tail connects to the 1st node (0-indexed).

method: 快慢指针

快指针一次走两格，慢指针一次走一格，如果有环，两个指针肯定会相遇

bool hasCycle(ListNode *head) {
    ListNode *fast = head;
    ListNode *slow = head;
    while (fast && fast->next) {
        fast = fast->next->next;    // 先走
        slow = slow->next;
        if (fast == slow) return true;  // 再判断
    }
    return false;
}

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LeetCode 142. Linked List Cycle II

LeetCode-142

Given a linked list, return the node where the cycle begins. If there is no cycle, return null.

There is a cycle in a linked list if there is some node in the list that can be reached again by continuously following the next pointer. Internally, pos is used to denote the index of the node that tail’s next pointer is connected to. Note that pos is not passed as a parameter.

Example 1:

Input: head = [3,2,0,-4], pos = 1
Output: tail connects to node index 1
Explanation: There is a cycle in the linked list, where tail connects to the second node.

method: 快慢指针

一个指针从快慢指针相遇地方开始遍历，一个从头开始遍历，相遇的节点就是环形的入口

ListNode *detectCycle(ListNode *head) {
    ListNode *fast = head;
    ListNode *slow = head;
    while (fast && fast->next) {
        fast = fast->next->next;
        slow = slow->next;
        if (slow == fast) { // 一个相遇节点开始走
            slow = head;    // 另一个从头开始走
            while (slow != fast) {
                slow = slow->next;
                fast = fast->next;
            }
            return fast;
        }
    }
    return nullptr;
}