java——力扣刷题小结

1、链表是否有环

官方解法：
哈希表

public class Solution {
    public boolean hasCycle(ListNode head) {
    //新建一个哈希表
        Set seen = new HashSet();
        //遍历
        while (head != null) {
        //根据哈希表性质，如果有节点添加失败则代表有地址重复，即
        //有相同的节点出现，即有环
            if (!seen.add(head)) {
                return true;
            }
            head = head.next;
        }
        //没找到即无环
        return false;
    }
}

个人解法：
快慢指针

public class Solution {
    public boolean hasCycle(ListNode head) {
    	//至少两个节点才有环
        if (head == null || head.next == null) {
            return false;
        }
        ListNode slow = head;
        ListNode fast = head.next;
        while (slow != fast) {
        	//如果快指针找到尾节点，则没有环
            if (fast == null || fast.next == null) {
                return false;
            }
            //满指针移动一次，快指针移动两次，若二者相遇即有环
            slow = slow.next;
            fast = fast.next.next;
        }
        return true;
    }
}

2、删除链表元素

class Solution {
    public ListNode removeElements(ListNode head, int val) {
        ListNode dummyHead = new ListNode(0);
        dummyHead.next = head;
        ListNode temp = dummyHead;
        while (temp.next != null) {
            if (temp.next.val == val) {
                temp.next = temp.next.next;
            } else {
                temp = temp.next;
            }
        }
        return dummyHead.next;
    }
}

3、反转链表

class Solution {
    public ListNode reverseList(ListNode head) {
        ListNode pre = null;
        ListNode cur = head;
        while(cur != null){
            ListNode next = cur.next;
            cur.next = pre;
            pre = cur;
            cur = next;
        }
        return pre;
    }
}