算法之环形链表

3.环形链表

3.1单链表是否有环

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

3.2已知单链表，求入环的节点

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