文章作者：Tyan
博客：noahsnail.com  |  CSDN  |  简书

1. Description

Merge k Sorted Lists

2. Solution

• Version 1

/**
 * 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) {
        int size = lists.size();
        if(size == 0) {
            return NULL;
        }
        ListNode* result = lists[0];
        for(int i = 1; i < size; i++) {
            result = mergeTwoLists(result, lists[i]);
        }
        return result;
    }
    
private:
    ListNode* mergeTwoLists(ListNode* first, ListNode* second) {
        ListNode* ptrFirst = first;
        ListNode* ptrSecond = second;
        ListNode* head = new ListNode(0);
        ListNode* current = head;
        while(ptrFirst && ptrSecond) {
            if(ptrFirst->val > ptrSecond->val) {
                current->next = ptrSecond;
                ptrSecond = ptrSecond->next;
            }
            else {
                current->next = ptrFirst;
                ptrFirst = ptrFirst->next;
            }
            current = current->next;
        }
        if(ptrFirst) {
            current->next = ptrFirst;
        }
        if(ptrSecond) {
            current->next = ptrSecond;
        }
        return head->next;
    }
    
};

• Version 2

/**
 * 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) {
        int size = lists.size();
        if(size == 0) {
            return NULL;
        }
        ListNode* result = NULL;
        while(lists.size() != 1) {
            ListNode * l1 = lists.back();
            lists.pop_back();
            ListNode * l2 = lists.back();
            lists.pop_back();
            result = mergeTwoLists(l1, l2);
            lists.insert(lists.begin(), result);
        }
        return lists[0];
    }
    
private:
    ListNode* mergeTwoLists(ListNode* first, ListNode* second) {
        ListNode* ptrFirst = first;
        ListNode* ptrSecond = second;
        ListNode* head = new ListNode(0);
        ListNode* current = head;
        while(ptrFirst && ptrSecond) {
            if(ptrFirst->val > ptrSecond->val) {
                current->next = ptrSecond;
                ptrSecond = ptrSecond->next;
            }
            else {
                current->next = ptrFirst;
                ptrFirst = ptrFirst->next;
            }
            current = current->next;
        }
        if(ptrFirst) {
            current->next = ptrFirst;
        }
        if(ptrSecond) {
            current->next = ptrSecond;
        }
        return head->next;
    }
    
};

Reference

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