快慢指针和链表原地反转

1，制作环型链表

2。检測链表中是否存在环（參考文章）

3。计算链表中环的长度

4， 计算链表中环起始的位置

5，推断一个链表是否存在回文，要求O（n）时间和O（1）空间的复杂度（推断链表是否是回文，要求O(n)时间。O(1)空间）

6。计算链表中间位置

7，链表原地反转（链表原地反转）

8，測试code

#include <iostream>
using namespace std;
/*
	@1: detect if there is a circule in a list -> detect_circule_exist
	@2: calculate the circule length in the list -> give_circ_len
	@3: locate the place where the circule starts -> locate_circ_place
	@4: detect if the list is palindrome -> time:o(n) and space:o(1)
*/
/*
	struct simple linked list
*/
struct item
{
	int n;
	struct item *next;
};
class iter {
	struct item *head;
	int circ_len;
public:
	void insert(int i);
	bool remove(int i);
	iter(){this->circ_len = 0;this->head = new(struct item);this->head->next = NULL;}
	bool create_circule(int i);
	void remove_circule();
	bool detect_circule_exist();
	int give_circ_len();
	int locate_circ_place();
	bool detect_palindrome();
	void display();
	~iter();
};
void iter::insert(int i) {
	struct item *cur;
	struct item *add_item = new(struct item);
	add_item->n = i;
	add_item->next = NULL;
	/* insert at tail */
	cur = this->head;
	while(cur->next) {
		cur = cur->next;
	}
	cur->next = add_item;
	cout << add_item <<endl;
}
bool iter::remove(int i)
{
	struct item *cur = this->head->next, *prev = this->head;
	while (cur) {
		if (cur->n == i) {
			prev->next = cur->next;
			return true;
		} else {
			cur = cur->next;
			prev = prev->next;
		}
	}
	return false;
}
void iter::display()
{
	struct item *cur = this->head->next;
	while (cur){
		cout << cur->n << " -> ";
		cur = cur->next;
	}
	cout <<"end" << endl;
}

/* create a circult at value: i, if not i exist return false*/
bool iter::create_circule(int i)
{
	struct item *cur = this->head;
	struct item *find = NULL;
	while (cur->next){
		cur = cur->next;
		if (cur->n == i) {
			find = cur;
		}
	}
	if (find) {
		cur->next = find;
		return true;
	} else
		return false;
}
/* detect if exist a circule in the linked list */
bool iter:: detect_circule_exist()
{
	/*quick and slow point to the first element of the linked list*/
	struct item *quick, *slow, *mark;
	bool find = false;
	slow = this->head->next;
	quick = this->head->next->next;

	while (quick && slow) {
		if (!find) {
			if (quick == slow) {
				find = true;
				mark = slow;
				slow = slow->next;
				this->circ_len++;
			} else {
				if (!quick->next || !quick->next->next)
					break;
				if (!slow->next)
					break;
				quick = quick->next->next;
				slow = slow->next;
			}
		} else {
			if (mark == slow) {
				return true;
			} else {
				slow = slow->next;
				this->circ_len++;
			}
		}
	}
	find = false;
	return find;

}
int iter:: give_circ_len()
{
	this->detect_circule_exist();
	return this->circ_len;
}
/* cal the len of non circule at the linked list */
int iter:: locate_circ_place()
{
	if (this->detect_circule_exist()) {
		struct item *quick, *slow;
		int i = this->circ_len;
		slow = this->head->next;
		quick = this->head->next;

		while (quick && i--) {
			quick = quick->next;
		}
		i = 0;
		while (quick && slow) {
			if (quick == slow) {
				return i;
			} else {
				i++;
				quick = quick->next;
				slow = slow->next;
			}
		}
	
	} else {
		return 0;
	}
}
void iter:: remove_circule()
{
	struct item *tail = this->head;
	struct item *mark;
	int i = this->locate_circ_place() + 1;
	while (tail->next && i--){
		tail = tail->next;
	}
	mark = tail;
	while (tail) {
			if (tail->next == mark) {
			tail->next = NULL;
			return;
		} else {
			tail = tail->next;
		}
	}
}
bool iter:: detect_palindrome()
{
	struct item *quick, *slow;
	/*first find the middle place of the linked list by quick and slow pointer*/
	slow = this->head->next;
	quick = this->head->next->next;
	while (slow) {
		struct item *mark;
		slow = slow->next;
		quick = quick->next->next;
		if (!quick) {
			/*reverse the linked list at the place*/
			mark = slow;
			quick = slow->next->next; /*q*/
			slow = slow->next; /*p*/
			struct item *temp;
			while (quick) {
				temp = quick->next;
				quick->next = slow;
				slow = quick;
				quick = temp;
			}
			mark->next->next = NULL;
			mark->next = slow;
			slow = this->head->next;
			mark = mark->next;
			while (slow && mark) {
				//cout << "odd first part: "<< slow->n << "addr:"  << slow << "second part: " << mark->n << "addr:" << mark <<endl;
				if (slow->n != mark->n)
					return false;
				else {
					slow = slow->next;
					mark = mark->next;
				}
			}
			return true;
		}
		if (!quick->next) {
			/*reverse the linked list at the place*/
			mark = slow;
			quick = slow->next->next; /*q*/
			slow = slow->next; /*p*/
			struct item *temp;
			while (quick) {
				temp = quick->next;
				quick->next = slow;
				slow = quick;
				quick = temp;
			}
			mark->next->next = NULL;
			mark->next = slow;
			slow = this->head->next;
			mark = mark->next;
			while (slow && mark) {
				//cout << "even first part: "<< slow->n << "addr:"  << slow << "second part: " << mark->n << "addr:" << mark <<endl;
				if (slow->n != mark->n)
					return false;
				else {
					slow = slow->next;
					mark = mark->next;
				}
			}
			return true;
		}
	}
}
iter::~iter()
{
	struct item *cur = this->head->next;
	if (this->detect_circule_exist())
		this->remove_circule();
	while (cur){
		struct item *temp = cur;
		cur = cur->next;
		delete temp;
		//cout <<"delete" << temp<< endl;
	}
	delete this->head;

}
int main()
{
	class iter myiter;
	/*
	for (int i = 0; i < 10; i++) {
		myiter.insert(i);
	}
	myiter.create_circule(2);
	
	if (myiter.detect_circule_exist())
		cout << "detect circule exist and circ len" << myiter.give_circ_len()<< endl;
	else
		cout << "no detect circule exist" <<endl;
 	cout << "circ point index" << myiter.locate_circ_place() <<endl;
 	
 	myiter.remove_circule();
 	myiter.display();
 	*/
 	/*
 	myiter.insert(1);
 	myiter.insert(2);
 	myiter.insert(3);
 	myiter.insert(3);
 	myiter.insert(2);
 	myiter.insert(1);
 	if (myiter.detect_palindrome())
 		cout << "even detect palindrome" <<endl;
 	else
 		cout << "even no detect palindrome" <<endl;
 	*/
 	myiter.insert(1);
 	myiter.insert(2);
 	myiter.insert(3);
 	myiter.insert(2);
 	myiter.insert(1);
 	if (myiter.detect_palindrome())
 		cout << "odd detect palindrome" <<endl;
 	else
 		cout << "odd no detect palindrome" <<endl;
 	return 1;
}