【leetcode】Reverse Linked List I & II

Reverse a singly linked list.

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Hint:
A linked list can be reversed either iteratively or recursively. Could you implement both?

 

class Solution {
public:
    ListNode* reverseList(ListNode* head) {
        if(head==NULL||head->next==NULL) return head;

        ListNode *p=head->next;
        ListNode *newhead=reverseList(head->next);
        head->next=NULL;
        p->next=head;
        return newhead;

       } 
};

 

II:

Reverse a linked list from position m to n. Do it in-place and in one-pass.

For example:
Given 1->2->3->4->5->NULL, m = 2 and n = 4,

return 1->4->3->2->5->NULL.

Note:
Given m, n satisfy the following condition:
1 ≤ m ≤ n ≤ length of list.

 

class Solution {
public:
    ListNode* reverseBetween(ListNode* head, int m, int n) {
        if(head==NULL||m<0||n<0) return head;
        if(head->next==NULL||m==n) return head;

        ListNode *p=head;
        int i=1;

        while(i<m)
        {
            p=p->next;
            i++;
        }

        for(int j=m;j<n;j++)
        {
            ListNode *q=p;
            for(int k=j;k<n;k++)
            {
                q=q->next;
            }
            swap(p->val,q->val);

            n--;
            p=p->next;
        }

        return head;

    }
};