1.删除指定元素

/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode() {}
 *     ListNode(int val) { this.val = val; }
 *     ListNode(int val, ListNode next) { this.val = val; this.next = next; }
 * }
 */
class Solution {
    public ListNode removeElements(ListNode head, int val) {
        ListNode dummyHead = new ListNode(0);
        dummyHead.next = head;
        ListNode p = dummyHead;
        while(p.next!=null){
            if(p.next.val==val) p.next=p.next.next;
            else p = p.next;
        }
        return dummyHead.next;
    }
}

2.设计链表

class ListNode{
    int val;
    ListNode next;
    public ListNode(int val){
        this.val = val;
    }   

}
class MyLinkedList {

    int size;
    ListNode head;

    public MyLinkedList() {
        size = 0;
        head = new ListNode(0); 
    }

    public int get(int index) {
        if(index<0||index>=size){
            return -1;
        }
        ListNode cur = head;
        for(int i=0;i<=index;i++) cur = cur.next;
        return cur.val;

    }

    public void addAtHead(int val) {
        addAtIndex(0,val);
    }

    public void addAtTail(int val) {
        addAtIndex(size,val);
    }

    public void addAtIndex(int index, int val) {
        if(index>size) return;
        index = Math.max(0,index);
        size++;
        ListNode pred = head;
        for(int i=0;i<index;i++) pred = pred.next;
        ListNode toAdd = new ListNode(val);
        toAdd.next = pred.next;
        pred.next = toAdd;

    }

    public void deleteAtIndex(int index) {
        if(index<0 || index>=size) return;
        size--;
        ListNode pred = head;
        for(int i=0;i<index;i++) pred=pred.next;
        pred.next = pred.next.next;

    }
}

/**
 * Your MyLinkedList object will be instantiated and called as such:
 * MyLinkedList obj = new MyLinkedList();
 * int param_1 = obj.get(index);
 * obj.addAtHead(val);
 * obj.addAtTail(val);
 * obj.addAtIndex(index,val);
 * obj.deleteAtIndex(index);
 */

3.反转链表

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

//递归
class Solution {
    public ListNode reverseList(ListNode head) {
     if(head==null||head.next=null) return head;
     //取出终极头结点
     ListNode new_head = reverseList(head.next);
     head.next.next = head;
     //就是给原第一个节点用的
     head.next = null;
     //祖传头结点
     return new_head;

    }

}

4.两两交换

class Solution {
    public ListNode swapPairs(ListNode head) {
        ListNode dummy = new ListNode(0);
        dummy.next = head;
        ListNode cur = dummy;
        while(cur.next!=null&&cur.next.next!=null){
            ListNode t = cur.next,t1 = cur.next.next.next;
            cur.next = cur.next.next;
            cur.next.next = t;
            cur.next.next.next = t1;
            cur = cur.next.next;
        }
        return dummy.next;


    }
}

5.删除倒数N个元素

class Solution {
    public ListNode removeNthFromEnd(ListNode head, int n) {
        ListNode dummy = new ListNode(0,head);
        ListNode slow = dummy,fast = dummy.next;
        for(int i=0;i<n;i++) fast = fast.next;
        while(fast!=null){
            slow = slow.next;
            fast = fast.next;
        }
        slow.next = slow.next.next;
        return dummy.next;
    }
}

6.链表相交

public class Solution {
    public ListNode getIntersectionNode(ListNode headA, ListNode headB) {
        ListNode p1 = headA,p2 = headB;
        while(p1!=p2){
            if(p1==null) p1 = headB;
            else p1 = p1.next;
            if(p2==null) p2 = headA;
            else p2 = p2.next;

        }
        return p1;

    }
}

7.链表有环&找到入口

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

    }
    if(hasCycle==true){
        slow = head;
        while(slow!=fast){
            slow = slow.next;
            fast = fast.next;
        }
        return fast;
    }
       return null; 
    }
}