【LeetCode】86. Merge k Sorted Lists

题目描述(Hard)

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

题目链接

https://leetcode.com/problems/merge-k-sorted-lists/description/

Example 1:

Input:
[
  1->4->5,
  1->3->4,
  2->6
]
Output: 1->1->2->3->4->4->5->6

算法分析

方法一：逐一合并；

方法二：分治两两合并；

提交代码：

/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     ListNode *next;
 *     ListNode(int x) : val(x), next(NULL) {}
 * };
 */
class Solution {
public:
    ListNode* mergeKLists(vector<ListNode*>& lists) {
        if (lists.empty()) return NULL;
        
        int interval = 1, len = lists.size();
        while (interval < len)
        {
            for (int i = 0; i < len - interval; i += 2 * interval)
                lists[i] = mergeTwoLists(lists[i], lists[i + interval]);
            interval *= 2;
        }
        
        return lists[0];
    }
    
    ListNode* mergeTwoLists(ListNode* list1, ListNode* list2) {
        ListNode *dummpy = new ListNode(-1);
        ListNode *p = dummpy;
        for (; list1 && list2; dummpy = dummpy->next)
        {
            if (list1->val < list2->val) { dummpy->next = list1; list1 = list1->next; }
            else { dummpy->next = list2; list2 = list2->next; }
        }
        
        dummpy->next = list1 != NULL ? list1 : list2;
        return p->next;
    }
};