leetcode - Linked List Cycle II

/**
* Definition for singly-linked list.
* struct ListNode {
*     int val;
*     ListNode *next;
*     ListNode(int x) : val(x), next(NULL) {}
* };
*/
class Solution {
public:
ListNode *detectCycle(ListNode *head) {
// IMPORTANT: Please reset any member data you declared, as
// the same Solution instance will be reused for each test case.
ListNode * itr = head;
ListNode * last = NULL;
int n = 1;
while (true){
if (itr==NULL)
return NULL;
ListNode * tmp = itr;
for (int i=0; i<n; i++){
tmp = tmp->next;
if (tmp==NULL)
return NULL;
else if (tmp == itr){
ListNode * rlt = head;
while (true){
ListNode * rltItr = rlt;
for (int j=0; j<=i; j++){
rltItr = rltItr->next;
if (rltItr == rlt)
return rlt;
}
rlt = rlt->next;
}
}
}
n++;
itr = itr->next;
}
return NULL;
}
};