LeetCode: Reorder List
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}.
地址：https://oj.leetcode.com/problems/reorder-list/
算法：先把链表从中间节点分成两段，然后将后半部分的链表逆置，最后在将后半部分的链表按题目的要求插入原链表中。代码：
1 /**
2 * Definition for singly-linked list.
3 * struct ListNode {
4 * int val;
5 * ListNode *next;
6 * ListNode(int x) : val(x), next(NULL) {}
7 * };
8 */
9 class Solution {
10 public:
11 void reorderList(ListNode *head) {
12 int num_node = 0;
13 ListNode *p = head;
14 while(p){
15 ++num_node;
16 p = p->next;
17 }
18 int half_num_node = (num_node + 1) / 2;
19 p = head;
20 ListNode *pre = NULL;
21 for (int i = 0; i < half_num_node; ++i){
22 pre = p;
23 p = p->next;
24 }
25 if(pre) pre->next = NULL;
26 pre = NULL;
27 ListNode *half_head = NULL;
28 while (p){
29 pre = p;
30 p = p->next;
31 pre->next = half_head;
32 half_head = pre;
33 }
34 p = head;
35 while(half_head){
36 pre = half_head;
37 half_head = half_head->next;
38 pre->next = p->next;
39 p->next = pre;
40 p = p->next->next;
41 }
42 }
43 };