链表相交、有环无环、交点等

struct int_List_Node{
	int value;
	struct int_List_Node *next;
};

//判断两个无环链表是否相交
bool is_crossed( struct int_List_Node * head1 , struct int_List_Node * head2 ) {
	if( ( head1 == NULL ) || ( head2 == NULL ) ) {
		printf( "error,empty pointer found\n");
	}
	struct int_List_Node * p , * q;
	p = head1;
	q = head2;
	while( p -> next != NULL ) {
		p = p -> next; 
	}
	while( q -> next != NULL ) {
		q = q -> next;
	}
	if( p == q ){
		return true;
	}
	return false;
}
//如果相交，求出第一个交点
struct int_List_Node * get_first_cross_point(  struct int_List_Node * head1 , struct int_List_Node * head2  ){
	struct int_List_Node * p , * q;
	p = head1;
	q = head2;
	while( ( p != NULL ) && ( q != NULL ) ) {
		p = p -> next;
		q = q -> next;
	}
	//当p指空，q非空的时候，让p指向head2，同步后指。这样q指空的时候，p向第一个公共节点
	if( p == NULL ) {
		p = head2;
		while( ( q != NULL ) ) {
			p = p -> next;
			q = q -> next;
		}
		return p -> next;
	}
	else if( q == NULL ) {
		q = head1;
		while( ( q != NULL ) ) {
			p = p -> next;
			q = q -> next;
		}
		return q -> next;
	}
}

//判断链表是否有环
bool is_cycled( struct int_List_Node * head ) {
	if( head == NULL ) {
		printf( "串是空的哦~\n");
		return false;
	}
	struct int_List_Node *p , *q;
	p = q = head;
	//p每次前移两个，q每次前移一个
	while( 1 ) {
		//注意这个条件先后，要先判断q是否为空，如果为空，则是不存在q -> next的
		if( ( q == NULL ) || ( q -> next == NULL ) ) {
			//这里不用判断p是否指空，因为q已经走过在p之前
			return false;
		}
		if( ( q == p ) || ( p -> next == q ) ) {
			//有环
			return false;
		}
		else {
			p = p -> next;
			q = q -> next -> next;
		}
	}
}

//有环链表和无环链表是不会相交的

//有环链表如何求第一个交点，还不知道怎么做