104. Merge K Sorted Lists

Solution:
1) Divide and Conquer:
/**
 * Definition for ListNode.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode(int val) {
 *         this.val = val;
 *         this.next = null;
 *     }
 * }
 */ 
public class Solution {
    /**
     * @param lists: a list of ListNode
     * @return: The head of one sorted list.
     */
     
    private ListNode mergeTwoLists(ListNode list1, ListNode list2) {
        if (list1 == null) return list2;
        if (list2 == null) return list1;
        
        
        ListNode rst = null;
        ListNode dummy = null;
        while(list1 != null && list2 != null) {
            int val = 0;
            if (list1.val < list2.val) {
                val = list1.val;
                list1 = list1.next;
            } else {
                val = list2.val;
                list2 = list2.next;
            }
            
            if(dummy == null) {
                dummy = new ListNode(val);
                rst = dummy;
            } else {
                dummy.next = new ListNode(val);
                dummy = dummy.next;
            }
            
        }
        
        if (list1 == null) dummy.next = list2;
        if (list2 == null) dummy.next = list1;
        
        return rst;
        
    }
    public ListNode mergeKLists(List<ListNode> lists) {  
        // write your code here
        if (lists.size() == 0) return null;
        if (lists.size() == 1) return lists.get(0);
        if (lists.size() == 2) return mergeTwoLists(lists.get(0), lists.get(1));
        return mergeTwoLists(
            mergeKLists(lists.subList(0, lists.size()/2)), 
            mergeKLists(lists.subList(lists.size()/2, lists.size()))
        );
    }
}