[LeetCode] Reorder List, Solution

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}.

[Thoughts]

目前想到的解法是，分三步来做：

1. 找出中间节点

2. 把中间节点之后的后半部分链表反序

3. 把前半部分链表及后半部分链表合并

 

[Code]

三步走解法

 1     void reorderList(ListNode *head) {
 2         if(head == NULL) return;
 3         // find the median node
 4         ListNode* fast = head;
 5         ListNode* slow = head;
 6         while(true)
 7         {
 8             fast = fast->next;
 9             if(fast == NULL)
10                 break;
11             fast = fast->next;
12             if(fast == NULL)
13                 break;
14             slow = slow->next;
15         }
16         
17         if(slow == NULL) return;
18         
19         // reverse second half of link list
20         ListNode* cur = slow;
21         ListNode* pre = slow->next;
22         cur->next = NULL;
23         while(pre!=NULL)
24         {
25             ListNode* temp = pre->next;
26             pre->next = cur;
27             cur = pre;
28             pre = temp;
29         }
30         
31         // merge two lists
32         ListNode* first = head;
33         ListNode* second = cur;
34         
35         while(second!= NULL&& first!=NULL && first!=second)
36         {
37             ListNode* temp = second->next;
38             second->next = first->next;
39             first->next = second;
40             first = second->next;
41             second = temp;
42         }
43     }

应该有更漂亮的解法，还在思考中。