LeetCode: Merge k Sorted Lists

/**
 * 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) {
        // Start typing your C/C++ solution below
        // DO NOT write int main() function
        int nSize = lists.size();
        if (nSize == 0) return NULL;
        if (nSize == 1) return lists[0];
        
        ListNode* result = new ListNode(0);
        ListNode* cur = result;
        ListNode* minNode = NULL;
        int empty = 0;
        while(empty < nSize)
        {
            int j = 0;
            int no = -1;
            for (int i = 0; i < nSize; ++i)
            {
                if (lists[i] != NULL)
                {
                    minNode = lists[i];
                    j = i + 1;
                    no = i;
                    break;
                }
                j = i + 1;
            }
            
            for (int k = j; k < nSize; ++k)
            {
                if (lists[k] != NULL && lists[k]->val < minNode->val)
                {
                    minNode = lists[k];
                    no = k;
                }
            }
            
            if (no < 0)
                return result->next;
                
            cur->next = minNode;
            cur = cur->next;
            
            lists[no] = lists[no]->next;
            if (lists[no] == NULL)
                ++empty;
        }
        
        return result->next;
    }
};