725. Split Linked List in Parts

package LeetCode_725

/**
 * 725. Split Linked List in Parts
 * https://leetcode.com/problems/split-linked-list-in-parts/description/
 *
 * Given a (singly) linked list with head node root, write a function to split the linked list into k consecutive linked list "parts".

The length of each part should be as equal as possible: no two parts should have a size differing by more than 1.
This may lead to some parts being null.

The parts should be in order of occurrence in the input list, and parts occurring earlier should always have a size greater than or equal parts occurring later.

Return a List of ListNode's representing the linked list parts that are formed.

Examples 1->2->3->4, k = 5 // 5 equal parts [ [1], [2], [3], [4], null ]
Example 1:
Input:
root = [1, 2, 3], k = 5
Output: [[1],[2],[3],[],[]]
Explanation:
The input and each element of the output are ListNodes, not arrays.
For example, the input root has root.val = 1, root.next.val = 2, 
oot.next.next.val = 3, and root.next.next.next = null.
The first element output[0] has output[0].val = 1, output[0].next = null.
The last element output[4] is null, but it's string representation as a ListNode is [].

Example 2:
Input:
root = [1, 2, 3, 4, 5, 6, 7, 8, 9, 10], k = 3
Output: [[1, 2, 3, 4], [5, 6, 7], [8, 9, 10]]
Explanation:
The input has been split into consecutive parts with size difference at most 1, and earlier parts are a larger size than the later parts.
Note:

The length of root will be in the range [0, 1000].
Each value of a node in the input will be an integer in the range [0, 999].
k will be an integer in the range [1, 50].
 * */

class ListNode(var `val`: Int) {
    var next: ListNode? = null
}

class Solution {
    /*
    * 1. calculate the len of the linked list
    * 2. calculate the length of each group
    * 3. calculate the count of extra node for each group
    * */
    fun splitListToParts(root_: ListNode?, k: Int): Array<ListNode?> {
        var len = 0
        var root = root_
        while (root != null) {
            len++
            root = root.next
        }

        val result = Array<ListNode?>(k, { null })
        val groupCount = len / k
        var extraNodeCount = len % k

        var head = root_
        var prev: ListNode? = null

        for (i in 0 until k) {
            result[i] = head
            var partCount = groupCount
            if (extraNodeCount > 0) {
                partCount++
            }
            extraNodeCount--
            //setting the group by scan from 0 to partCount
            for (j in 0 until partCount) {
                prev = head
                //move the pointer
                head = head?.next
            }
            //now,head is the HEAD of next group,
            //prev is the last of this group,so break it in list,
            //for example: 1,2,3,4,5 => [1,2,3,4]
            if (prev != null) {
                prev.next = null
            }
        }

        return result
    }
}