LeetCode OJ:Sort List

Sort List

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Sort a linked list in O(n log n) time using constant space complexity.

算法思想：

nlogn复杂度内排序，可以采用快拍或者归并排序，这里采用归并排序实现；

思想很简单：先将一条整链分成两条链，对这两条链进行归并

/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     ListNode *next;
 *     ListNode(int x) : val(x), next(NULL) {}
 * };
 */
class Solution {
public:
    ListNode *sortList(ListNode *head){
		
		ListNode *left=head,*right=head;
		if(left==NULL||left->next==NULL)return left;

		while(right->next!=NULL&&right->next->next!=NULL){
			left=left->next;
			right=right->next->next;
		}
		if(left->next!=NULL){
			right=left->next;
			left->next=NULL;
			left=head;
		}
		return merge(sortList(left),sortList(right));
	}
	ListNode * merge(ListNode *left,ListNode *right){
		ListNode *list=new ListNode(0);
		ListNode *cur;
		cur=list;
		while(left!=NULL&&right!=NULL){
			if(left->val<=right->val){
				cur->next=left;
				cur=cur->next;
				left=left->next;
			}
			else{
				cur->next=right;
				cur=cur->next;
				right=right->next;
			}
		}
		if(left!=NULL){
			cur->next=left;
			
		}
		if(right!=NULL){
			cur->next=right;
			
		}
		return list->next;
	}
};