MergeKSortedLists.java

//Solution to https://leetcode.com/problems/merge-k-sorted-lists/
//Approach: We use the logic behind merging two sorted lists to merge k number of lists in sorted order. 
//Pair up k lists and merge each pair.
//After the first pairing, k lists are merged into k/2 lists with average 2N/k length, then k/4, k/8 and so on.
//This is repeated until we get the final sorted linked list.

class Solution {
    public ListNode mergeTwoLists(ListNode l1, ListNode l2) {
    ListNode h = new ListNode(0);
    ListNode ans=h;
    while (l1 != null && l2 != null) {
        if (l1.val < l2.val) {
            h.next = l1;
            h = h.next;
            l1 = l1.next;
        } else {
            h.next = l2;
            h = h.next;
            l2 = l2.next;
        }
    }
    if(l1==null){
        h.next=l2;
    }
    if(l2==null){
        h.next=l1;
    } 
    return ans.next;
}
public ListNode mergeKLists(ListNode[] lists) {
    if(lists.length==0){
        return null;
    }
    int interval = 1;
    while(interval<lists.length){
        System.out.println(lists.length);
        for (int i = 0; i + interval< lists.length; i=i+interval*2) {
            lists[i]=mergeTwoLists(lists[i],lists[i+interval]);            
        }
        interval*=2;
    }

    return lists[0];
}
}