MergeKSortedLists_MergeTwo.cpp

/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     ListNode *next;
 *     ListNode(int x) : val(x), next(NULL) {}
 * };
 */
class Solution {
public:
    ListNode* mergeTwoLists(ListNode* l1, ListNode* l2){
        ListNode head(0);
        ListNode* h = &head;
        while(l1&&l2){
            if(l1->val < l2->val){
                h->next = l1;
                h=h->next;
                l1= l1->next;
            }
            else{
                h->next = l2;
                h = h->next;
                l2=l2->next;
            }
        }
        if(l1)
        {
            h->next = l1;
        }
        if(l2)
        {
            h->next = l2;
        }
        
        return head.next;
    }

    ListNode* mergeKLists(vector<ListNode*>& lists) {
        int len_l = lists.size();
        if(len_l == 0)
            return nullptr;
        
        int step = 1;
        while(step < len_l){
            for(int i = 0; i + step < len_l; i = i + step*2)
                {
                    lists[i] = mergeTwoLists(lists[i], lists[i+step]);
                }
            step = step*2;
        }
        
        return lists[0];
    }
};