LeetCode--Reorder List

Given a singly linked list L: L0→L1→…→Ln-1→Ln,

reorder it to: L0→Ln→L1→Ln-1→L2→Ln-2→…

You must do this in-place without altering the nodes' values.

For example,

Given

{1,2,3,4}

, reorder it to

{1,4,2,3}

.

/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     ListNode *next;
 *     ListNode(int x) : val(x), next(NULL) {}
 * };
 */
class Solution 
{
public:
    void reorderList(ListNode *head) 
    {
        if(head== NULL || head->next==NULL)
            return;
        ListNode* temp = head;
        vector<ListNode*> res;
        while(temp!=NULL)
        {
            res.push_back(temp);
            temp = temp->next;
        }
        int n = res.size()-1;
        int low = 0;
        int high = n;
        while(low<high)
        {
            res[low]->next = res[high];
            res[high]->next = res[low+1];
            low++;
            high--;
        }
        res[low]->next = NULL;
        head = res[0];
    }
};

还可以通过链表直接改进空间复杂度降为常数