leetcode No143. Reorder List

Question:

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}.

Algorithm:

用栈，这样栈的出栈顺序就是倒序，而遍历链表是正序

Accepted Code：

/** * 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* pNode1=head;    //L0 L1 L2 ...        stack<ListNode*> s;        int count=0;        while(pNode1){            s.push(pNode1);            pNode1=pNode1->next;            count++;        }        pNode1=head;        ListNode* pNode=head;        count--;        pNode1=pNode->next;        while(count){            pNode->next=s.top();            s.pop();            pNode=pNode->next;            count--;            if(count==0)                break;            pNode->next=pNode1;            pNode=pNode->next;            pNode1=pNode->next;            count--;        }        pNode->next=NULL;    }};