Approach

# 走訪linked-list，注意交換數值的順序
Traverse the linked-list, paying attention to the order of value swaps.

Time Complexity

# n = length of list
O(n)

Space Complexity

# n = length of list
O(n)

Code

/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     ListNode *next;
 *     ListNode() : val(0), next(nullptr) {}
 *     ListNode(int x) : val(x), next(nullptr) {}
 *     ListNode(int x, ListNode *next) : val(x), next(next) {}
 * };
 */
class Solution {
public:
    ListNode* reverseList(ListNode* head) {
        ListNode *prev = NULL, *cur = head, *next = NULL;

        while (cur != NULL) {
            next = cur -> next;
            cur -> next = prev;
            prev = cur;
            cur = next;
        }

        return prev;
    }
};