LeetCode刷题笔记（链表）：merge-k-sorted-lists

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题目描述

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

解题思路

之前做过一道题是对两个排序链表进行合并，这次是k个。所以，需要在原来的基础长做一些封装。

按照归并排序的思想，我们通过二分法得到中点进行递归，自下而上进行两两归并，直到只剩最后一个链表。

C++版代码实现

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/**
 * 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.size() == 0)
            return NULL;
        return mergeList(lists, 0, lists.size()-1);
    }
    
    ListNode *mergeList(vector<ListNode *> &lists, int low, int high){
        if(high <= low)
            return lists[low];
        int mid = (low + high) / 2;
        ListNode *left = mergeList(lists, low, mid);
        ListNode *right = mergeList(lists, mid+1, high);
        return merge(left, right);
    }
    
    ListNode *merge(ListNode *left, ListNode *right) {
        if(left == NULL)
            return right;
        if(right == NULL)
            return left;
        
        ListNode *dummy = new ListNode(0);
        ListNode *cur = dummy;
        while(left != NULL && right != NULL){
            if(left->val < right->val){
                cur->next = left;
                left = left->next;
            }
            else{
                cur->next = right;
                right = right->next;
            }
            cur = cur->next;
        }
        cur->next = left ? left : right;
        return dummy->next;
    }
};

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