https://leetcode.com/problems/merge-k-sorted-lists/

Solution.swift

// Time: O(NxK)
// Space: O(NxK) - need to store the results somewhere
class Solution {
    func mergeKLists(_ lists: [ListNode?]) -> ListNode? {
        var tops = lists.compactMap { $0 ?? nil }
        var resultRoot: ListNode?
        var resultLast: ListNode?
        while tops.count > 0 {
            var pickIdx = 0
            var val = Int.max

            // search
            for i in 0..<tops.count {
                if tops[i].val < val { 
                    pickIdx = i 
                    val = tops[i].val
                }
            }

            // save
            let newNode = ListNode(tops[pickIdx].val)
            if resultRoot != nil {
                resultLast?.next = newNode
                resultLast = newNode
            } else {
                resultRoot = newNode
                resultLast = newNode
            }

            // advance
            if let newTop = tops[pickIdx].next {
                tops[pickIdx] = newTop
            } else {
                tops.remove(at: pickIdx)
            }
            
        }
        return resultRoot
    }
}