[leetcode]Linked List Cycle

Linked List Cycle

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

Follow up:
Can you solve it without using extra space?

算法

思路1：

快慢指针，当两个指针相遇，则表示有环，否则则无环，可能需要多遍遍历，空间复杂度O(1)，时间复杂度O(n)

public class Solution {
    public boolean hasCycle(ListNode head) {
        if(head == null) return false;
        ListNode fast = head;
        ListNode slow = head;
        while(true){
            if(fast.next == null || fast.next.next == null) return false;
            fast = fast.next.next;
            slow = slow.next;
            if(fast == slow) return true;
        }
    }
}

思路2：

hash表，当某个节点有两个前驱，则该节点是环的起点，而且只需一次遍历，空间复杂度O(n)，时间复杂度O(n)

public class Solution {
    public boolean hasCycle(ListNode head) {
        if(head == null) return false;
        ListNode tem = new ListNode(0);
        tem.next = head;
        Map<ListNode,ListNode> hash = new HashMap<ListNode,ListNode>();
        while(true){
            if(tem.next == null) return false;
            if(hash.get(tem.next) != null) return true;
            hash.put(tem.next,tem);
            tem = tem.next;
        }
    }
}

思路２同样是Linked List Cycle II的解法

ListNode的结构如下：

public class ListNode {
    int val;
    ListNode next;
    ListNode(int x){
        val = x;
        next = null;
    }
}