206_ReverseLinkedList

---

title: 206. Reverse Linked List

tags:
- Linked List categories: leetcode comments: false

problem

reverse linked list

solution

option 1 - iterative

需要三個個指標

class Solution {
public:
    ListNode* reverseList(ListNode* head) {
        if(!head || !head->next) return head;
        ListNode *pre = nullptr, *cur = head, *post = head->next;
        while(post){
            cur->next = pre;
            pre = cur;
            cur = post ;
            post = post->next;
        }
        cur->next = pre;
        return cur;
    }
};

• another version

class Solution {
public:
    ListNode* reverseList(ListNode* head) {
        if(!head || !head->next) return head;
        ListNode *pre = nullptr, *cur = head;
        while(cur->next){
            ListNode *post = cur->next;
            cur->next = pre;
            pre = cur;
            cur = post ;
        }
        cur->next = pre;
        return cur;
    }
};

• other version

class Solution {
public:
    ListNode* reverseList(ListNode* head) {
        if(!head || !head->next) return head;
        ListNode *pre = new ListNode(-1), *cur = head;
        pre->next = head;
        while(cur->next){
            ListNode *post = cur->next;
            cur->next = post->next;
            post->next = pre->next;
            pre->next = post;
        }
        return pre->next;
    }
};

option 2 - recursive

class Solution {
public:
    ListNode* reverseList(ListNode* head) {
        if(!head) return nullptr;
        if(!head->next) return head;
        ListNode *node = reverseList(head->next);
        head->next->next = head;
        head->next = nullptr;;
        return node;
        
    }
};

analysis

• time complexity O(n)
• solution complexity O(n)