Merge Two Sorted Lists

初阶。

Merge two sorted linked lists and return it as a new list. The new list should be made by splicing together

the nodes of the first two lists.

进阶。

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

简洁的方式，使用递归：

public:    ListNode* Merge(ListNode* pHead1, ListNode* pHead2)    {        if (pHead1 == NULL)        return pHead2;        if (pHead2 == NULL)            <span style="white-space:pre"></span>return pHead1;        if (pHead1->val < pHead2->val)        {            pHead1->next = Merge(pHead1->next,pHead2);            return pHead1;        }else        {            pHead2->next = Merge(pHead1,pHead2->next);            return pHead2;        }            }};

#include <iostream>#include <vector>using namespace std;struct LinkNode{int data;LinkNode *next;LinkNode(int x):data(x),next(nullptr){}};LinkNode* mergeTwoList(LinkNode *l1,LinkNode *l2){LinkNode *p1 = l1,*p2 = l2,*head,*pre;LinkNode *dummy = new LinkNode(-1);pre = dummy;while(p1&&p2){if(p1->data < p2->data){pre->next = p1;p1 = p1->next;}else{pre->next = p2;p2 = p2->next;}pre = pre->next;}if(p1)        pre->next = p1;if(p2)        pre->next = p2;head = dummy->next;delete dummy;return head;}//merge k个listLinkNode* mergeKList(vector<LinkNode*> &lists){if(lists.size() == 0) return NULL;LinkNode *p = lists[0];for(unsigned int i=1;i<lists.size();i++){p = mergeTwoList(p,lists[i]);}return p;}//对链表初始化LinkNode *initLink(LinkNode *dummy,int array[],int len){LinkNode *head,*tmp;//dummy表示哑节点，head一般表示头指针，它指向第一个节点。tmp = dummy;//用尾插法for(int i = 0;i < len;i++){LinkNode *p = new LinkNode(array[i]);tmp->next = p;tmp = tmp->next;}head = dummy->next;delete dummy;return head;}//显示listvoid showList(LinkNode *head){while(head){printf("%d ",head->data);head = head->next;}printf("\n");}//摧毁链表void destroyList(LinkNode *head){LinkNode *dummy = new LinkNode(-1);dummy->next = head;LinkNode *pre,*cur,*tmp;pre = dummy;cur = pre->next;while(cur){tmp = cur->next;pre->next = tmp;delete cur;cur = tmp;}delete dummy;printf("The list has been destroyed!\n");}void main(){LinkNode *dummy1 = new LinkNode(-1);int array1[] = {1,3,5,7,9};int len1 = sizeof(array1)/sizeof(int);LinkNode *head1 = initLink(dummy1,array1,len1);showList(head1);LinkNode *dummy2 = new LinkNode(-2);int array2[] = {2,4,6,8,10,11,12};int len2 = sizeof(array2)/sizeof(int);LinkNode *head2 = initLink(dummy2,array2,len2);showList(head2);LinkNode *dummy3 = new LinkNode(-3);int array3[] = {2,7,10,13,15,20,26,33,100,120};int len3 = sizeof(array2)/sizeof(int);LinkNode *head3 = initLink(dummy3,array3,len3);showList(head3);//LinkNode *head = mergeTwoList(head1,head2);//showList(head);vector<LinkNode*> lists;lists.push_back(head1);lists.push_back(head2);lists.push_back(head3);LinkNode *head = mergeKList(lists);showList(head);destroyList(head);}