2017-09-11 LeetCode_025 Reverse Nodes in k-Group

25. Reverse Nodes in k-Group

Given a linked list, reverse the nodes of a linked list k at a time and return its modified list.
k is a positive integer and is less than or equal to the length of the linked list. If the number of nodes is not a multiple of k then left-out nodes in the end should
remain as it is.
You may not alter the values in the nodes, only nodes itself may be changed.
Only constant memory is allowed.
For example,

Given this linked list: 

1->2->3->4->5

For k = 2, you should return: 

2->1->4->3->5

For k = 3, you should return: 

3->2->1->4->5

solution:

class Solution {

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public:

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   ListNode* reverseKGroup(ListNode* head, int k) {

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       int d = 0;

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       ListNode *p = head;

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       while (p != NULL) {

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           d++; p = p->next;

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}

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if (d < k) return head;

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if (k <= 1) return head;

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p = head;

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ListNode *q = head->next, *r = head->next->next;

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int k_ = k-2;

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while (k_--) {

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q->next = p;

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p = q; q = r; r = r->next;

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}

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q->next = p;

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head->next = reverseKGroup(r, k);

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return q;

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   }

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};