LeetCode OJ:Reorder List

Reorder List

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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}.

算法思想：用栈处理比较方便

/**
 * 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)return;
		stack<ListNode *> st;
		ListNode *p=head,*q=head;
		while(q!=NULL&&q->next!=NULL){
			p=p->next;
			q=q->next->next;
		}
		q=p->next;
		p->next=NULL;
		while(q!=NULL){
			st.push(q);
			q=q->next;
		}
		p=head;
		ListNode *t;
		while(p!=NULL){
			t=p;
			p=p->next;
			if(!st.empty()){
				t->next=st.top();
				st.top()->next=p;
				st.pop();
			}
		}
	}
};