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;
        }
        
    }
};