mergeTwoSortList

mergeTwoSortList

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.

/** * Definition for singly-linked list. * struct ListNode { *     int val; *     ListNode *next; *     ListNode(int x) : val(x), next(NULL) {} * }; */class Solution {public:    ListNode *mergeTwoLists(ListNode *l1, ListNode *l2) {        // 应该先保存头结点的问题！        ListNode *dummy = new ListNode(0);        ListNode *List = dummy;        while (l1 && l2){            if (l1->val < l2->val){                List ->next= l1;                l1  = l1-> next;            } else {                List ->next = l2;                l2 = l2 -> next ;            }            List = List -> next;        }        List->next =  (NULL != l1) ? l1 : l2;        return dummy->next;    }};

对于k个list：

• 可以循环调用上述的函数

ListNode *mergeKLists(vector<ListNode *> &lists) {  if(lists.size() == 0) return NULL;  ListNode *p = lists[0];  for(int i =1; i< lists.size(); i++)    {     p = mergeTwoSortList(p, lists[i]);    }   return p;}

• 网上看到也可以使用堆?

class Solution {public:    // comparison structure, required by priority_queue template    struct greaterListNode{        bool operator() (ListNode* x, ListNode* y) {return x->val > y->val;}    };    ListNode *mergeKLists(vector<ListNode *> &lists) {        if(lists.size() == 0)            return NULL;        if(lists.size() == 1)            return lists[0];        ListNode* dummy = new ListNode(-1);        ListNode* pre = dummy;        std::priority_queue<ListNode*, vector<ListNode*>, greaterListNode> pq;        // Insert entries of every list into heap        for(auto& lst:lists)            if(lst != NULL)                pq.push(lst);        // Recursively parse every element in lists        while (!pq.empty()) {            pre->next = pq.top();            pre = pre->next;            pq.pop();            if (pre != NULL && pre->next != NULL)                pq.push(pre->next);        }        ListNode* head = dummy->next;        delete dummy;        return head;    }};