Problem Statement

You are given an array of k linked-lists lists, each linked-list is sorted in ascending order.

Merge all the linked-lists into one sorted linked-list and return it.

Example 1:

Input: lists = [[1,4,5],[1,3,4],[2,6]]
Output: [1,1,2,3,4,4,5,6]
Explanation: The linked-lists are:
[
  1->4->5,
  1->3->4,
  2->6
]
merging them into one sorted list:
1->1->2->3->4->4->5->6

Example 2:

Input: lists = []
Output: []

Example 3:

Input: lists = [[]]
Output: []

Constraints:

• k == lists.length
• 0 <= k <= 10^4
• 0 <= lists[i].length <= 500
• 10^4 <= lists[i][j] <= 10^4
• lists[i] is sorted in ascending order.
• The sum of lists[i].length won't exceed 10^4.

Problem Link

Merge k Sorted Lists - LeetCode

Code

/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     ListNode *next;
 *     ListNode() : val(0), next(nullptr) {}
 *     ListNode(int x) : val(x), next(nullptr) {}
 *     ListNode(int x, ListNode *next) : val(x), next(next) {}
 * };
 */
#include<iterator>
class Solution {
public:
    ListNode* mergeKLists(vector<ListNode*>& lists) {
        
        multimap<int,ListNode*> m;
        
        for(auto list:lists)
        {
            while(list)
            {
                m.insert({list->val,list}) ;
                list = list->next;
            }
        }
        
       
        ListNode *temp = new ListNode();
        ListNode *head = temp;
        for(auto itr:m)
        {
           temp->next = itr.second;
           temp = temp->next; 
        }
        
        
        
        return head->next;
        
    }
};