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

merge-k-sorted-lists.cpp

/**
 * 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 nullptr;
        size_t delta = 1;
        while (delta < lists.size()) 
        {
            for (int i=0; i<lists.size()-delta; i=i+2*delta)
                lists[i] = mergeTwoLists(lists[i], lists[i+delta]);
            delta *= 2;
        }
        
        return lists[0];
    }
    
    ListNode* mergeTwoLists(ListNode* l1, ListNode* l2)
    {
        if (l1 == NULL) return l2;
        if (l2 == NULL) return l1;

        auto v1 = l1->val;
        auto v2 = l2->val;
        auto root = v1 <= v2 ? l1 : l2;
        v1 <= v2 ? (l1 = l1->next) : (l2 = l2->next);

        auto cur = root;
        while (l1 != NULL && l2 != NULL)
        {
            if (l1->val <= l2->val)
            {
                cur->next = l1;
                l1 = l1->next;
            }
            else
            {
                cur->next = l2;
                l2 = l2->next;
            }
            cur = cur->next;
        }
        cur->next = l1 != NULL ? l1 : l2;

        return root;
    }
};

Runtime: 80 ms, faster than 31.01% of C++ online submissions for Merge k Sorted Lists.
Memory Usage: 10.9 MB, less than 90.17% of C++ online submissions for Merge k Sorted Lists.

Runtime: 28 ms, faster than 90.64% of C++ online submissions for Merge k Sorted Lists.
Memory Usage: 11.8 MB, less than 43.30% of C++ online submissions for Merge k Sorted Lists.

Runtime: 24 ms, faster than 99.23% of C++ online submissions for Merge k Sorted Lists.
Memory Usage: 10.8 MB, less than 93.34% of C++ online submissions for Merge k Sorted Lists.