206. Reverse Linked List

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problem

reverse linked list

solution

option 1 - iterative

需要三個個指標

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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 
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    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
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    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

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    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)