lintcode merge-k-sorted-lists 合并k个排序链表

问题描述

lintcode

笔记

用到了堆/优先队列
代码1用到了STL的priority_queue
代码2用到了STL的heap，要特别注意push_heap,pop_heap的实际操作，我就被坑了一上午的时间。比如说，如果是小顶堆的话，pop_heap是把要删除的元素（最小的元素）放到了最后，然后再用vec.pop_back()才能正确地删除。还有cmp要写成cmp()。
代码3自己写了个heap

代码1 priority_queue

/** * Definition of ListNode * class ListNode { * public: *     int val; *     ListNode *next; *     ListNode(int val) { *         this->val = val; *         this->next = NULL; *     } * } */class Solution {public:    /**     * @param lists: a list of ListNode     * @return: The head of one sorted list.     */    struct cmp    {        bool operator() (ListNode* a, ListNode* b)        {            return ((a->val) > (b->val));        }    };    ListNode *mergeKLists(vector<ListNode *> &lists) {        // write your code here        priority_queue<ListNode*, vector<ListNode*>, cmp> q;        ListNode* dummy = new ListNode(0);        ListNode* iter = dummy;        for (int i = 0; i < lists.size(); i++)        {            ListNode* nowNode = lists[i];            if (nowNode)            {                q.push(nowNode);            }        }        while (!q.empty())        {            ListNode* nowNode = q.top();            q.pop();            iter->next = nowNode;            iter = iter->next;            if (nowNode->next)                q.push(nowNode->next);        }        iter->next = NULL;        return dummy->next;    }};

代码2 STL heap

/** * Definition of ListNode * class ListNode { * public: *     int val; *     ListNode *next; *     ListNode(int val) { *         this->val = val; *         this->next = NULL; *     } * } */class Solution {public:    /**     * @param lists: a list of ListNode     * @return: The head of one sorted list.     */    struct cmp    {        bool operator () (ListNode* a, ListNode* b)        {            return a->val > b->val;        }    };    ListNode *mergeKLists(vector<ListNode *> &lists) {        // write your code here        vector<ListNode*> mylist;        for (int i = 0; i < lists.size(); i++)        {            if (lists[i])                mylist.push_back(lists[i]);        }        ListNode* dummy = new ListNode(0);        ListNode* iter = dummy;        make_heap(mylist.begin(), mylist.end(), cmp());        while (!mylist.empty())        {            pop_heap(mylist.begin(), mylist.end(), cmp());            ListNode* node = mylist.back();            iter->next = node;            iter = iter->next;            mylist.pop_back();            if (node->next)            {                mylist.push_back(node->next);                push_heap(mylist.begin(), mylist.end(), cmp());            }        }        iter->next = NULL;        return dummy->next;    }};

代码3 自制heap

/** * Definition of ListNode * class ListNode { * public: *     int val; *     ListNode *next; *     ListNode(int val) { *         this->val = val; *         this->next = NULL; *     } * } */void vecp(vector<ListNode*> v){    for (int i = 0; i < v.size(); i++)        cout << v[i]->val << ' ';    cout << endl;}class Solution {public:    /**     * @param lists: a list of ListNode     * @return: The head of one sorted list.     */    ListNode *mergeKLists(vector<ListNode *> &lists) {        // write your code here        ListNode* dummy = new ListNode(0);        ListNode* iter = dummy;        vector<ListNode*> mylist;        for (int i = 0; i < lists.size(); i++)        {            if (lists[i])                mylist.push_back(lists[i]);        }        makeMinHeap(mylist, mylist.size()-1);        // vecp(mylist);        while (!mylist.empty())        {            swap(mylist[0], mylist[mylist.size()-1]);            ListNode* node = mylist.back();            iter->next = node;            iter = iter->next;            mylist.pop_back();            minHeapFixDown(mylist, 0, mylist.size()-1);            if (node->next)            {                mylist.push_back(node->next);                minHeapFixUp(mylist, mylist.size()-1);            }        }        iter->next = NULL;        return dummy->next;    }    void minHeapFixDown(vector<ListNode*> &vec, int start, int end)    {        int par = start;        int minChild = par * 2 + 1;        while (minChild <= end)        {            if (minChild+1 <= end && vec[minChild]->val > vec[minChild+1]->val)                minChild++;            if (vec[minChild]->val >= vec[par]->val)                return;            // vecp(vec);            swap(vec[minChild], vec[par]);            // cout << "asd\n"            // vecp(vec);            par = minChild;            minChild = par * 2 + 1;        }    }    void makeMinHeap(vector<ListNode*> &vec, int end)    {        for (int i = (end - 1) / 2; i >= 0; i--)            minHeapFixDown(vec, i, end);    }    void minHeapFixUp(vector<ListNode*> &vec, int end)    {        int child = end;        int par = (end - 1) / 2;        while (par >= 0)        {            if (vec[par]->val <= vec[child]->val)// 应该是<=而不是<,否则会死循环                return;            swap(vec[par], vec[child]);            child = par;            par = (child - 1) / 2;        }    }};