LeetCode:Reorder List

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

reorder it to: L₀→L_n→L₁→L_n-1→L₂→L_n-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}.

解題思路:

用棧將整個鏈表存儲，然後將棧頂元素插入到棧底元素的後面，循環多少次呢？鏈表

的長度除2即可.

解題代碼:

/**
 * 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)
    {
        stack stk;
        ListNode *tmp = head;
        int cnt = 0 ;
        while(tmp)
        {
            stk.push(tmp);
            tmp = tmp->next;
            ++cnt;
        }
        tmp = head ;
        for(int i = 1 ; i <= cnt / 2 ; ++i)
        {
            ListNode *tmp1 = stk.top();
            stk.pop();
            tmp1->next = tmp->next ;
            tmp->next = tmp1 ;
            tmp = tmp1->next;
        }
        if(head)
            tmp->next = NULL ;
    }
};

注：上述代碼的空間復雜度是O(n),不過這題應該可以做到O(1)的空間的,遍歷鏈表從中間分割即可.