题目
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}.
解题之法
/**
* 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 || !head->next || !head->next->next) return;
ListNode *fast = head;
ListNode *slow = head;
while (fast->next && fast->next->next) {
slow = slow->next;
fast = fast->next->next;
}
ListNode *mid = slow->next;
slow->next = NULL;
ListNode *last = mid;
ListNode *pre = NULL;
while (last) {
ListNode *next = last->next;
last->next = pre;
pre = last;
last = next;
}
while (head && pre) {
ListNode *next = head->next;
head->next = pre;
pre = pre->next;
head->next->next = next;
head = next;
}
}
};
分析
这道链表重排序问题可以拆分为以下
使用快慢指针来找到链表的中点，并将链表从中点处断开，形成两个独立的链表。
将第二个链翻转。
将第二个链表的元素间隔地插入第一个链表中。
