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}
.
本题 O(n)空间时间做完,但实际有O(1)的做法。
那个办法是把后半部分翻转,然后merge一下就可以了。
/** * 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<ListNode *> sk; if(head == NULL)return; int num=1; ListNode *temp = head; while(temp ->next != NULL) { sk.push(temp->next); temp =temp->next; num++; } temp = head; if(num<3)return; for(int i = 0 ; i < num/2 ;i++) { ListNode *last =sk.top(); sk.pop(); if(!sk.empty()){ ListNode *n = sk.top(); n->next =NULL; } last->next = temp->next; temp->next = last; temp = last->next; } } };