Tuesday, May 12, 2015

LeetCode [206] Reverse Linked List

Reverse a singly linked list.
Example:
Input: 1->2->3->4->5->NULL
Output: 5->4->3->2->1->NULL
Follow up:
A linked list can be reversed either iteratively or recursively. Could you implement both?

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/**
 * 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 || !(head->next)) return head;
        ListNode* H = new ListNode(0);
        H->next = head;
        ListNode* p = head->next;
        head->next = NULL;
        while(p)
        {
            ListNode* q = p->next;
            p->next = H->next;
            H->next = p;
            p = q;
        }
        return H->next;
    }
};

//C++: method1 14ms
/**
 * 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) {
        ListNode *H = new ListNode(0);
        while(head){
            ListNode *first = H->next;
            H->next = head;
            head = head->next;
            H->next->next = first;
        }
        return H->next;
    }
};
]]></script>

<script class="brush: js" type="syntaxhighlighter"><![CDATA[
//C++: method2 12ms
/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     ListNode *next;
 *     ListNode(int x) : val(x), next(NULL) {}
 * };
 */
class Solution {
public:
    void helper(ListNode* head, ListNode* H){
        if(!head) return;
        ListNode* first = H->next;
        H->next = head;
        ListNode* next = head->next;
        H->next->next = first;
        helper(next, H);
    }

    ListNode* reverseList(ListNode* head) {
        ListNode *H = new ListNode(0);
        helper(head, H);
        return H->next;
    }
};
]]></script>

<script class="brush: js" type="syntaxhighlighter"><![CDATA[
//C++: method3 8ms
/**
 * 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 || !head->next) return head;
        ListNode* newHead = reverseList(head->next);
        head->next->next = head;
        head->next = NULL;
        return newHead;
    }
};


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/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode() {}
 *     ListNode(int val) { this.val = val; }
 *     ListNode(int val, ListNode next) { this.val = val; this.next = next; }
 * }
 */
class Solution {
    public ListNode reverseList(ListNode head) {
        ListNode H = new ListNode();
        ListNode p = head;
        while(p!=null){
            ListNode h = H.next;
            ListNode next = p.next;
            p.next = h;
            H.next = p;
            p = next;
        }
        return H.next;
    }
}
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