java链表的常见简单面试算法题详解

 /**
     * 头插法
     * @param head
     * @return
     */
    public static Node head_insert(Node head, int t){
        Node node=new Node(t);
        node.setNext(head.getNext());
        head.setNext(node);
        return head;
    }

    /**
     * 尾插法
     * @param head
     * @return
     */
    public static Node tail_insert(Node head,int t){
        Node tail=head;
        Node target=new Node(t);
        while (tail.getNext()!=null){
            tail=tail.getNext();
        }
        tail.setNext(target);
     return head;
    }
}

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

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

    }
}

public class Solution {
    public ListNode detectCycle(ListNode head) {
        Boolean result=false;
        ListNode slow=head;
        ListNode fast=head;
        while (slow!=null&&fast!=null&&fast.next!=null){
           slow=slow.next;
           fast=fast.next.next;
           if(slow==fast){
               result=true;
               break;
           }
        }
        if(result==false){
         return null;
        }
        slow=head;
        while (slow!=fast){
            slow=slow.next;
            fast=fast.next;
        }
        return slow;
    }
}

class Solution {
    public ListNode mergeTwoLists(ListNode list1, ListNode list2) {
     ListNode head=new ListNode(0);
     ListNode pre=head;
     while(list1!=null&&list2!=null){
        if(list1.val<list2.val){
          pre.next=list1;
          pre=list1;
          list1=list1.next;
        }else{
          pre.next=list2;
          pre=list2;
          list2=list2.next;
        }
     }
     if(list1==null){
     pre.next=list2;
     }else{
        pre.next=list1;
     }
     return head.next;
   }
}