链表 leetcode 143 Reorder List

原题链接：Reorder List

题解：/**
* 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) {
/*
思路：使用辅助栈将中间位置之后的节点保存起来，然后依次插入就能达到目标；
Time Complexity:O(n)
Space Complexity:O(n)
*/
if(!head || !head->next || !head->next->next) return;
ListNode* res=head;
ListNode* last=NULL;
stack<ListNode*>sta;
int count=0;
while(head){
count++;
head=head->next;
}
int k=count%2==1?(count+1)/2:count/2;
head=res;
for(int i=1;i<=count;i++){
if(i==k)last=head;
if(i>k)sta.push(head);
head=head->next;
}
last->next=NULL;
head=res;
while(!sta.empty()){
sta.top()->next=head->next;
head->next=sta.top();
sta.pop();
head=head->next->next;
}
}
};