23. Merge k Sorted Lists

class Solution {
    public ListNode mergeKLists(ListNode[] lists) {
        if(lists == null || lists.length == 0){
            return null;
        }
        return helper(lists,0,lists.length-1);
    }
    public ListNode helper(ListNode[] lists , int start ,int end){
        if(start > end ){
            return null;
        }
        if(start == end){
            return lists[start];//here start can be any thing start or end;
        }
        int mid = start + (end - start)/2;
        ListNode left = helper(lists, start , mid);
        ListNode right = helper(lists, mid + 1, end);
        return merge(left , right);
        
    }
    public ListNode merge(ListNode l1 , ListNode l2){
        ListNode temp = new ListNode(-1);
        ListNode sol = temp;
        while (l1 != null && l2 != null){
            if(l1.val > l2.val){
                temp.next = l2;
                l2 = l2.next;
                temp = temp.next;
            }
            else {
                temp.next = l1;
                l1 = l1.next;
                temp = temp.next;
            }
        }
        if(l1 != null || l2 != null){
            temp.next = l1 != null ? l1 : l2; 
        }
        return sol.next;
    }
}