LeetCode第23题之Merge k Sorted Lists

楼主是参考：http://www.cnblogs.com/skysand/p/4300711.html，感觉这篇博客写得挺好的。

C++代码：

#include <vector>#include <iostream>#include <algorithm>using namespace std;/** * Definition for singly-linked list. */  struct ListNode {      int val;      ListNode *next;      ListNode(int x) : val(x), next(NULL) {}  };class Solution {public:    // 小顶堆,以第i个元素为根建立小顶堆    //位置从1开始，取元素时记得-1    // 自顶向下    void minHeap(vector<ListNode *> &heap, int i)    {        int l = i*2;        int r = i*2+1;        int least = i;        //如果左孩子存在，且左孩子小于父节点        if (l-1<heap.size() && heap[l-1]->val < heap[least-1]->val)        {            least = l;        }        if (r-1 < heap.size() && heap[r-1]->val < heap[least-1]->val)        {            least = r;        }        //如果父节点的值不是最小        if (least != i)        {            swap(heap[i-1], heap[least-1]);            minHeap(heap, least);        }    }    // 建立小顶堆    // 自低向上    void makeHeap(vector<ListNode *> &heap)    {        int sz = heap.size();        for (int i=sz/2;i>0;--i)        {            minHeap(heap,i);        }    }    //弹出堆顶的元素    ListNode *popHeap(vector<ListNode *>& heap)    {        int sz = heap.size();        if (sz > 0)        {            swap(heap[0], heap[sz-1]);        }        ListNode * p = heap.back();        heap.pop_back();        minHeap(heap,1);        return p;    }     // 在小顶堆中插入一个元素    // 自低向上    void pushHeap(vector<ListNode *> &heap, ListNode *p)    {        heap.push_back(p);        int child = heap.size();        int parent = child/2;        while (parent && heap[child-1]->val < heap[parent-1]->val)        {            swap(heap[child-1], heap[parent-1]);            child = parent;            parent /= 2;        }    }    ListNode* mergeKLists(vector<ListNode*>& lists) {         // 使用堆排序,         // 1. 选出每个链表的头来插入小顶堆中，        // 2. 再把堆顶接入合并链表中，        // 3. 被选出的指针后移再加入小顶堆中,回到2        // 4. 最后所有链表都为空时，返回合并链表的头指针        int sz = lists.size();        if (0 == sz) return NULL;        vector<ListNode *> heap;        // 1. 选出每个链表的头来插入小顶堆中，        for (int i=0;i<sz;++i)        {            if (lists[i])                 heap.push_back(lists[i]);        }        makeHeap(heap);        /*for (int i=0;i<heap.size();++i)        {            cout<<heap[i]->val<<'\t';        }        cout<<endl;*/         // 2. 再把堆顶接入合并链表中，        ListNode head(-1);        ListNode *p = &head;        ListNode *tmp = NULL;        while(!heap.empty())        {            tmp = popHeap(heap);            p->next = tmp;            p = p->next;            // 3. 被选出的指针后移再加入小顶堆中,回到2            if (tmp->next)            {                pushHeap(heap, tmp->next);            }        }         // 4. 最后所有链表都为空时，返回合并链表的头指针        return head->next;    }};void print_list(vector<ListNode *> lists){    for (int i=0;i<lists.size();++i)    {        ListNode *p = lists[i];        while(p)        {            cout<<p->val<<'\t';            p = p->next;        }        cout<<endl;    }}int main(){    Solution s;    vector<ListNode *> lists;    for (int i=0;i<4;++i)    {        ListNode *head,*p;        head = NULL;        for (int j=5;j>i;--j)        {            p = head;            head = new ListNode(j);            head->next = p;    //      head = p;        }        lists.push_back(head);      }//  print_list(lists);    ListNode * res = s.mergeKLists(lists);    while(res)    {        cout<<res->val<<'\t';        res = res->next;    }    cout<<endl;    return 0;}

**优化代码：

后来想到既然堆每次加入一个元素的时候都要调整堆顶，那么每次把要添加的元素换到堆顶再调整就不用写pushHeap的函数了，当要添加的元素为空时，相当于执行popHeap函数，因此可以简化代码：**

#include <vector>#include <iostream>#include <algorithm>using namespace std;/** * Definition for singly-linked list. */  struct ListNode {      int val;      ListNode *next;      ListNode(int x) : val(x), next(NULL) {}  };class Solution {public:    // 小顶堆,以第i个元素为根建立小顶堆    //位置从1开始，取元素时记得-1    // 自顶向下    void minHeap(vector<ListNode *> &heap, int i)    {        int l = i*2;        int r = i*2+1;        int least = i;        //如果左孩子存在，且左孩子小于父节点        if (l-1<heap.size() && heap[l-1]->val < heap[least-1]->val)        {            least = l;        }        if (r-1 < heap.size() && heap[r-1]->val < heap[least-1]->val)        {            least = r;        }        //如果父节点的值不是最小        if (least != i)        {            swap(heap[i-1], heap[least-1]);            minHeap(heap, least);        }    }    void makeHeap(vector<ListNode *> &heap)    {        int sz = heap.size();        for (int i=sz/2;i>0;--i)        {            minHeap(heap,i);        }    }    ListNode* mergeKLists(vector<ListNode*>& lists) {        int sz = lists.size();        if (0 == sz) return NULL;        vector<ListNode *> heap;        //将每条链表的首节点的指针加入到heap堆中        for (int i=0;i<sz;++i)        {            if (lists[i])                 heap.push_back(lists[i]);        }        makeHeap(heap);        /*for (int i=0;i<heap.size();++i)        {            cout<<heap[i]->val<<'\t';        }        cout<<endl;*/        ListNode head(-1);        ListNode *p = &head;        ListNode *tmp = NULL;        while(!heap.empty())        {            tmp = heap[0];            p->next = tmp;            p = tmp;            if (tmp->next)            {                heap[0] = tmp->next;                    }            else            {                swap(heap[0],heap[heap.size()-1]);                heap.pop_back();            }            minHeap(heap,1);        }        return head->next;    }};void print_list(vector<ListNode *> lists){    for (int i=0;i<lists.size();++i)    {        ListNode *p = lists[i];        while(p)        {            cout<<p->val<<'\t';            p = p->next;        }        cout<<endl;    }}int main(){    Solution s;    vector<ListNode *> lists;    for (int i=0;i<4;++i)    {        ListNode *head,*p;        head = NULL;        for (int j=5;j>i;--j)        {            p = head;            head = new ListNode(j);            head->next = p;    //      head = p;        }        lists.push_back(head);      }//  print_list(lists);    ListNode * res = s.mergeKLists(lists);    while(res)    {        cout<<res->val<<'\t';        res = res->next;    }    cout<<endl;    return 0;}