# Simple Java Solution with Merge Sort and in-place sort two lists

• ``````public class Solution {
public ListNode mergeKLists(ListNode[] lists) {
if (lists.length == 0) return null;
return mergeKLists(lists,0,lists.length-1);
}

public ListNode mergeKLists(ListNode[] lists, int pre, int post) {
if (pre == post) return lists[pre];
ListNode a = lists[pre];
ListNode b = lists[post];
if (post - pre >= 2) {
int middle = pre + (post - pre) / 2;
a = mergeKLists(lists,pre,middle);
if (middle+1 != post) b = mergeKLists(lists,middle+1,post);
}
return mergeLists(a,b);
}

public ListNode mergeLists(ListNode a, ListNode b) {
if (a == null) return b;
if (b == null) return a;
if (a.val > b.val) {
b.next = mergeLists(a,b.next);
return b;
}
else {
a.next = mergeLists(a.next,b);
return a;
}
}
}
``````

The code divides the array into smaller pieces until there are only two ListNode left. Then it will sort these two arrays recursively.

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