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# 143. Reorder List
## 1.問題
* **給予一個linked list, 重新排序**
## 2.想法
* 提問
* function header, parameter
* test input
* 觀察
* 說明想法
* 這類題目看起來很複雜, 重點是要仔細分析後熟悉linked list的運算
* 將list平分成兩段
* 將list 2逆轉
* 將list 2依序插入list 1中
* 測試計算複雜度
## **3.程式碼**
```
/**
* 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) {
return;
}
//Find middle point
ListNode* slow = head;
ListNode* fast = head;
while (fast->next && fast->next->next) {
slow = slow->next;
fast = fast->next->next;
}
ListNode* mid = slow->next;
slow->next = NULL;
//Reverse second part
ListNode* last = mid, *pre = NULL;
while (last) {
ListNode *next = last->next;
last->next = pre;
pre = last;
last = next;
}
//Inset the second list into first list
while (head && pre) {
ListNode *next = head->next;
head->next = pre;
pre = pre->next;
head->next->next = next;
head = next;
}
}
};
```
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