LeetCode:Merge k Sorted Lists

Merge k Sorted Lists

Total Accepted: 82424  Total Submissions: 354076  Difficulty: Hard

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

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Hide Tags   Divide and Conquer Linked List Heap Hide Similar Problems   (E) Merge Two Sorted Lists (M) Ugly Number II

思路：

归并排序，与数组归并排序类似

/**
 * 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;
        return mergeSort(lists, 0, lists.size()-1);
    }
    
    ListNode *mergeSort(vector<ListNode*>& lists, int s, int e){
        
        if(s==e) return lists[s];
        else {
            int mid = (s + e)>>1;
            ListNode *l1 = mergeSort(lists, s, mid);
            ListNode *l2 = mergeSort(lists, mid+1, e);
            return merge(l1,l2);
        }
    }
    
    ListNode *merge(ListNode *l1, ListNode *l2){
        
        if(l1==NULL) return l2;
        if(l2==NULL) return l1;
        ListNode dummy(0); // 伪头结点
        ListNode *p = &dummy;
        
        while(l1 && l2){
            if(l1->val < l2->val){
                p->next = l1;
                l1=l1->next;
            }
            else {
                p->next = l2;
                l2=l2->next;
            }
            p = p->next;
        }
        
        p->next = l1!=NULL?l1:l2;
        return dummy.next;
    }
    
};