206. Reverse Linked List

1.問題

• 反轉linked list

2.想法

• 提問

  • 確認題意: 反轉linked list

• function header, parameter

• test input

• 說明想法

  • 三個指標

    • current: 指向head, 下一回合時移到next

    • next: 指向head->next, 並讓current移動

    • pre: 一開始指向NULL, 並讓head->next移動

• 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:
    ListNode* reverseList(ListNode* head) {
        if (head == NULL)
        {
            return NULL;
        }
        
        ListNode *prev = head;
        ListNode *curr = head->next;
        prev->next = NULL;
        
        while (curr != NULL)
        {
            ListNode *next = curr->next;
            curr->next = prev;
            prev = curr;
            curr = next;
        }
        return prev;
    }
};

4.Performance