[LeetCode] 092: Rotate List

[Problem]

Given a list, rotate the list to the right by k places, where k is non-negative.

For example:
Given 1->2->3->4->5->NULL and k = 2,
return 4->5->1->2->3->NULL.

[Solution]

class Solution {
public:
ListNode *rotateRight(ListNode *head, int k) {
// Note: The Solution object is instantiated only once and is reused by each test case.

// invalid
if(NULL == head || k <= 0)return head;

// get the length of the list
int len = 0;
ListNode* p = head;
while(p != NULL){
len++;
p = p->next;
}

// reset k
k %= len;
if(k == 0)return head;

// move k steps forward
p = head;
for(int i = 0; i < k; ++i){
p = p->next;
}

// cut the list into two parts, and link then reversed
ListNode *prep = p;// the node before p;
ListNode *q = head;// the node two be cut
ListNode *preq = q;// the node before q;
while(p != NULL){
preq = q;
q = q->next;
prep = p;
p = p->next;
}

// link
preq->next = NULL;
prep->next = head;
head = q;

return head;
}
};

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