标签：back lis end 有序链表 优化 klist while tor return
合并 k 个排序链表，返回合并后的排序链表。请分析和描述算法的复杂度。
首先我想到的是两两合并，分析时间大概是接近O(n^2)了。leecode显示需要200ms.
/** * 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 NULL; if(lists.size() == 1) { return lists[0]; } ListNode prehead = ListNode(-1); ListNode *pre = &prehead; ListNode* p1 = lists[lists.size()-1]; lists.pop_back(); ListNode* p2 = mergeKLists(lists); while(p1 && p2) { if(p1->val < p2->val) { pre->next = p1; //pre = pre->next; p1 = p1->next; } else { pre->next = p2; //pre = pre->next; p2 = p2->next; } pre = pre->next; } pre->next = p1?p1:p2; return prehead.next; } };
后来借鉴了快速排序的思想。优化了一点时间，能够达到48ms。感觉还不是很满意。
/** * 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 NULL; if(lists.size() == 1) { return lists[0]; } if(lists.size() == 2) { return merge2Lists(lists[0], lists[1]); } int mid = lists.size() /2; vector<ListNode*> left; vector<ListNode*> right; left.insert(left.begin(), lists.begin(), lists.begin()+mid); right.insert(right.begin(), lists.begin()+mid, lists.end()); ListNode* leftNode = mergeKLists(left); ListNode* rightNode = mergeKLists(right); return merge2Lists(leftNode, rightNode); } ListNode* merge2Lists(ListNode*l1, ListNode*l2){ ListNode prehead = ListNode(-1); ListNode *pre = &prehead; //ListNode* p1 = lists[lists.size()-1]; //lists.pop_back(); //ListNode* p2 = mergeKLists(lists); ListNode *p1 = l1; ListNode *p2 = l2; while(p1 && p2) { if(p1->val < p2->val) { pre->next = p1; //pre = pre->next; p1 = p1->next; } else { pre->next = p2; //pre = pre->next; p2 = p2->next; } pre = pre->next; } pre->next = p1?p1:p2; return prehead.next; } };
标签：back lis end 有序链表 优化 klist while tor return
原文地址：https://www.cnblogs.com/Shinered/p/11291647.html
