23. Merge k Sorted Lists - jiejackyzhang/leetcode-note GitHub Wiki

Merge k sorted linked lists and return it as one sorted list. Analyze and describe its complexity.

解题思路为divide and conquer。 把k个lists拆分为两部分，然后转化为Merge two sorted linked lists问题来解。

/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode(int x) { val = x; }
 * }
 */
public class Solution {
    
    private ListNode mergeTwoLists(ListNode l1, ListNode l2) {
        if(l1 == null) return l2;
        if(l2 == null) return l1;
        if(l1.val > l2.val) {
            l2.next = mergeTwoLists(l1, l2.next);
            return l2;
        } else {
            l1.next = mergeTwoLists(l1.next, l2);
            return l1;
        }
    }
    
    private ListNode partition(ListNode[] lists, int low, int high) {
        if(low > high) return null;
        if(low == high) return lists[low];
        int mid = (low + high) / 2;
        ListNode l1 = partition(lists, low, mid);
        ListNode l2 = partition(lists, mid+1, high);
        return mergeTwoLists(l1, l2);
    }
    
    public ListNode mergeKLists(ListNode[] lists) {
        if(lists == null) return null;
        return partition(lists, 0, lists.length-1);
        
    }
}