Leetcode 1669: Merge In Between Linked Lists

In this Leetcode problem, we want to merge a list list2 into another list list1 and remove some elements of list1 where list2 is going.

Problem statement

Solution

We want to simply build the list simply by going to the node in list1 where list2 should be inserted. Then keep going in list1 to get to the node that should come after the last node of list2.

Let’s look at an example:

list1 = [1, 2, 3, 4, 5];
list2 = [6, 7, 8];
a = 3;
b = 4;

We setup a list current to go through list1. After we go through the list to get the last node before insertion, we have:

list1 = [1, 2, 3, 4, 5];
list2 = [6, 7, 8];
a = 3;
b = 4;
current = [2, 3, 4, 5];

Then, after we de-reference current to list2 and remember the toRemove node:

list1 = [1, 2, 6, 7, 8];
list2 = [6, 7, 8];
a = 3;
b = 4;
current = [2, 6, 7, 8];
toRemove = [3, 4, 5];

Then, we find the node to add back at the end of list2, and add it at the end of list2:

list1 = [1, 2, 6, 7, 8, 5];
list2 = [6, 7, 8, 5];
a = 3;
b = 4;
current = [2, 6, 7, 8, 5];
toRemove = [5];

Finally, this is the solution code, step by step:

class Solution {
    public ListNode…