LInkedList

When using two pointers, do not use 

fast.next != null

fast may be null

Given a sorted linked list, delete all nodes that have duplicate numbers, leaving only distinct numbers from the original list.

For example,
Given 1->2->3->3->4->4->5, return 1->2->5.

Given 1->1->1->2->3, return 2->3.

Recursion

 public ListNode deleteDuplicates2(ListNode head) {        if(head == null || head.next == null){            return head;        }        int val = head.val;        ListNode next = head.next;        if(next.val != val){            head.next = deleteDuplicates(next);            return head;        }else{            while(next != null && next.val == val){                next = next.next;            }            return deleteDuplicates(next);        }    }

Sort a linked list using insertion sort.

public static ListNode insertionSortList(ListNode head) {if( head == null ){            return head;        }        ListNode dummy = new ListNode(0); //new starter of the sorted list        ListNode cur = head; //the node will be inserted        ListNode pre = dummy; //insert node between pre and pre.next        ListNode next = null; //the next node will be inserted        //not the end of input list        while( cur != null ){            next = cur.next;            //find the right place to insert            while( pre.next != null && pre.next.val < cur.val ){                pre = pre.next;            }            //insert between pre and pre.next            cur.next = pre.next;            pre.next = cur;            pre = dummy;            cur = next;        }        return dummy.next;    }

When you are using dummy.next.next you'd better create a new reference node

Merge two sorted list

//This function is from Merge Two Sorted Lists.public static ListNode merge(ListNode l1,ListNode l2){    if(l1==null) return l2;    if(l2==null) return l1;    if(l1.val<l2.val){        l1.next=merge(l1.next,l2);        return l1;    }else{        l2.next=merge(l1,l2.next);        return l2;    }}

Reverse LinkedList

public static ListNode reverseListDummy(ListNode head){if(head == null){return head;}ListNode dummy = new ListNode(-1);ListNode cur = head;while(cur != null){ListNode temp = cur.next;cur.next = dummy.next;dummy.next = cur;cur = temp;}return dummy.next;}