LeetCode-Merge k Sorted Lists

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

/**
 * 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
        if(lists.size()==0)return NULL;
        int min,index;
         int i=0;
        for(;i<lists.size();i++){
            if(lists[i]!=NULL){
                min=lists[i]->val;
                index=i;
                break;
            }
        }
        if(i==lists.size())return NULL;
        i++;
        if(i<lists.size())
        for(;i<lists.size();i++){
            if(lists[i]!=NULL&&lists[i]->val<min){
                min=lists[i]->val;
                index=i;
            }
        }
        ListNode* root=lists[index];
        lists[index]=lists[index]->next;
        ListNode* tail=root;
        while(true){
            int i=0;
            for(;i<lists.size();i++){
                if(lists[i]!=NULL){
                    min=lists[i]->val;
                    index=i;
                    break;
                }
            }
            if(i==lists.size())return root;
            i++;
            if(i<lists.size())
            for(;i<lists.size();i++){
                if(lists[i]!=NULL&&lists[i]->val<min){
                    min=lists[i]->val;
                    index=i;
                }
            }
            tail->next=lists[index];
            tail=tail->next;
            lists[index]=lists[index]->next;
        }
    }
};