I was working on LeetCode to practice myself with Java programming. I encountered a question about insertion sort a linked list. My code currently runs correctly, but it failed on an instance with more than 8000 nodes. I guessed there must be a bottleneck for my code and I need your help. My code : /** * Definition for singly-linked list. * public class ListNode { *     int val; *     ListNode next; *     ListNode(int x) { val = x; } * } */public class Solution {    public ListNode insertionSortList(ListNode head) {        if(head == null) {            return null;        }        ListNode dummy = new ListNode(0);        dummy.next = head;        ListNode in; // i's next        ListNode jn; // j's next        ListNode inn; // in's next        // I was using i.next and j.next as the current node. I hope by doing so I can get the node before the current node.         for(ListNode i = head; i.next != null; i = i.next) {             for(ListNode j = dummy; j.next != i.next; j = j.next) {                if(i.next.val < j.next.val) {                     in = i.next; // recording necessary node                     jn = j.next;                     inn = in.next;                    if(j.next == i) { // when i and j are adjacent                        j.next = in;                        in.next = jn;                        jn.next = inn;                            i = in; // back step i when finish exchange                    } else { // different strategy when they are not                        j.next = in;                        in.next = jn;                        i.next = inn;                        i = jn; // back step i when finish exchange                     }                    break;                 }            }        }        return dummy.next;    }}Its debugging information:Input: 17 -> 15 -> 14 -> 14 -> 12 -> 11 -> 10 -> 10 -> 9 -> 8Dummy: 0 -> 15 -> 17 -> 14 -> 14 -> 12 -> 11 -> 10 -> 10 -> 9 -> 8Dummy: 0 -> 14 -> 15 -> 17 -> 14 -> 12 -> 11 -> 10 -> 10 -> 9 -> 8Dummy: 0 -> 14 -> 14 -> 15 -> 17 -> 12 -> 11 -> 10 -> 10 -> 9 -> 8Dummy: 0 -> 12 -> 14 -> 14 -> 15 -> 17 -> 11 -> 10 -> 10 -> 9 -> 8Dummy: 0 -> 11 -> 12 -> 14 -> 14 -> 15 -> 17 -> 10 -> 10 -> 9 -> 8Dummy: 0 -> 10 -> 11 -> 12 -> 14 -> 14 -> 15 -> 17 -> 10 -> 9 -> 8Dummy: 0 -> 10 -> 10 -> 11 -> 12 -> 14 -> 14 -> 15 -> 17 -> 9 -> 8Dummy: 0 -> 9 -> 10 -> 10 -> 11 -> 12 -> 14 -> 14 -> 15 -> 17 -> 8Dummy: 0 -> 8 -> 9 -> 10 -> 10 -> 11 -> 12 -> 14 -> 14 -> 15 -> 17Result: 8 -> 9 -> 10 -> 10 -> 11 -> 12 -> 14 -> 14 -> 15 -> 17I found a similar implementation as to mine, I don't know why his is faster: public class Solution {public ListNode insertionSortList(ListNode head) {    ListNode helper=new ListNode(0);    ListNode pre=helper;    ListNode current=head;    while(current!=null) {        pre=helper;        while(pre.next!=null&&pre.next.val<current.val) {            pre=pre.next;        }        ListNode next=current.next;        current.next=pre.next;        pre.next=current;        current=next;    }    return helper.next;}The input I 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give me some help, I thought this question was easy and then I spent two days testing it. And it is quite disappointing when I got Time limit exceed error.
Time limit exceeded for java insertion sort linked list
java;linked list;time limit exceeded;insertion sort
The culprit is i = jn; in the last else branch.This is not needed because you know that the entire list from head to i should be sorted and you need to continue from the old i.next.Take the following step:a->b->d->f-> ... y->z->c->h   ^                ^   j                ithis is where you do the insert which results ina->b->c->d->f->... y->z->h   ^     ^   j     iThis causes you to need to repeat a full scan for every time you need to advance i. This can easily cause the time complexity to rise to O(n^3) for a reverse sorted list.You should have.a->b->c->d->f->... y->z->h   ^               ^   j               iThe way to solve this would be to not advance i in the outer for loop so you can keep it pointing to z after the insertion. This also means that you can use the same insertion strategy in both cases    for(ListNode i = head; i.next != null; ) {         bool inserted = false;        for(ListNode j = dummy; j.next != i.next; j = j.next) {            if(i.next.val < j.next.val) {                 in = i.next; // recording necessary node                 jn = j.next;                 inn = in.next;                j.next = in;                in.next = jn;                i.next = inn;                   inserted=true;                break;             }        }        if(!inserted) i = i.next; //i.next was already in the correct place->advance i    }