[LeetCode] 053: Merge K Sorted Lists

最新推荐文章于 2019-10-31 10:48:55 发布

原创最新推荐文章于 2019-10-31 10:48:55 发布 · 3.5k 阅读

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本文提供了一种有效的算法来合并多个已排序的链表，并返回一个单一的排序链表。该方法首先定义了一个用于合并两个链表的辅助函数，然后通过递归地将链表分成更小的部分进行合并。

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

[Solution]

/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     ListNode *next;
 *     ListNode(int x) : val(x), next(NULL) {}
 * };
 */
class Solution {
public:
	/**
	 * merge two lists
	 */
	ListNode* mergeLists(ListNode* first, ListNode* second){
		// at least one of the two lists is null
		if(NULL == first)return second;
		if(NULL == second)return first;

		// both of them are not null
		ListNode *head = NULL;
		ListNode *p1 = first;
		ListNode *p2 = second;
		if(first->val < second->val){
			head = new ListNode(first->val);
			p1 = p1->next;
		}
		else{
			head = new ListNode(second->val);
			p2 = p2->next;
		}
		
		// merge
		ListNode *p = head;
		while(p1 != NULL || p2 != NULL){
			if(NULL == p1){
				p->next = new ListNode(p2->val);
				p2 = p2->next;
			}
			else if(NULL == p2){
				p->next = new ListNode(p1->val);
				p1 = p1->next;
			}
			else if(p1->val < p2->val){
				p->next = new ListNode(p1->val);
				p1 = p1->next;
			}
			else{
				p->next = new ListNode(p2->val);
				p2 = p2->next;
			}
			p = p->next;
		}
		return head;
	}
	/**
	 * merge lists[begin, end]
	 */
	ListNode *mergeKLists(vector<ListNode *> &lists, int begin, int end){
		// invalid
		if(begin > end || begin < 0 || begin >= lists.size() || end >= lists.size())return NULL;

		// only one list
		if(begin == end)return lists[begin];

		// two lists
		if(begin+1 == end)return mergeLists(lists[begin], lists[end]);

		// more than two lists
		int mid = begin + (end-begin)/2;
		return mergeLists(mergeKLists(lists, begin, mid), mergeKLists(lists, mid+1, end));
	}
	/**
	 * merge k lists
	 */
	ListNode *mergeKLists(vector<ListNode *> &lists) {
		// Start typing your C/C++ solution below
		// DO NOT write int main() function
		return mergeKLists(lists, 0, lists.size()-1);
	}
};