LeetCode 23. Merge k Sorted Lists

原题链接在这里：https://leetcode.com/problems/merge-k-sorted-lists/

题目：

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

题解：

是Merge Two Sorted Lists的进阶版，现在是k条，用divide and conquer, 两条两条合并 最后合并成一个大的。

Note: mergeSort中别忘了最后的返回值是lists[l].

分析时间复杂度T(k) = 2T(k/2) + O(nk). k = lists.length, n是原数组中最长的list的长度。根据master theorem, Time Complexity: O(nk*logk).

Space: O(logk) 是recursion tree的高度。

AC Java: 

/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode(int x) { val = x; }
 * }
 */
public class Solution {
    public ListNode mergeKLists(ListNode[] lists) {
        if(lists == null || lists.length == 0){
            return null;
        }
        return mergeSort(lists, 0, lists.length-1);
    }
    
    private ListNode mergeSort(ListNode[] lists, int l, int r){
        if(l == r){
            return lists[l];
        }
        int m = l+(r-l)/2;
        ListNode leftList = mergeSort(lists, l, m);
        ListNode rightList = mergeSort(lists, m+1, r);
        return merge(leftList, rightList);
    }
    
    private ListNode merge(ListNode l1, ListNode l2){
        if(l1 == null){
            return l2;
        }
        
        ListNode dummy = new ListNode(0);
        ListNode cur = dummy;
        while(l1 != null && l2 != null){
            if(l1.val <= l2.val){
                cur.next = l1;
                l1 = l1.next;
                cur = cur.next;
            }else{
                cur.next = l2;
                l2 = l2.next;
                cur = cur.next;
            }
        }
        if(l1 != null){
            cur.next = l1;
        }
        if(l2 != null){
            cur.next = l2;
        }
        return dummy.next;
    }
}