C/C++经典面试题之判断链表是否有环

#include <cstdio>

typedef struct list
{
int data;
struct list *next;
}LIST;

/* Method 1: check the occurrence of p->next from head to p */
bool check_circle_1(LIST *head)
{
LIST *p = head, *q= NULL;

if (p == NULL)
return false;

while (p->next)
{
/* check whether p points to itself */
if (p->next == p)
{
return true;
}

/* check the occurrence of p->next in head to p */
q = head;
while (q != p)
{
if (q == p->next)
{
return true;
}
q = q->next;
}
p = p->next;
}
/* p->next is NULL, not a circle */
return false;
}

/* Method 2: q goes faster than p, if at last p == q, means there's circle */
/* 优点：逻辑上简单。 缺点：无法具体知道从哪个点拆开该圈 */
bool check_circle_2(LIST *head)
{
LIST *p, *q;
p = head;

if (p == NULL)
return false;

q = p->next;

while (p != NULL && q != NULL)
{
if (p == q)
{
return true;
}
p = p->next;
if (q->next == NULL)
{
return 0;
}
else
{
q=q->next->next;
}
}
return 0;
}
void testIS()
{
LIST a, b, c, *head = &a;
a.next = &b;
b.next = &c;
c.next = &a;
printf("1:is circle | 0:not circle || result check_circle_1(head):\
%d\n", check_circle_1(head));
printf("1:is circle | 0:not circle || result check_circle_1(head):\
%d\n", check_circle_2(head));
}
void testNO()
{
LIST a, b, c, *head = &a;
a.next = &b;
b.next = &c;
c.next = NULL;
printf("1:is circle | 0:not circle || result check_circle_1(head):\
%d\n", check_circle_1(head));
printf("1:is circle | 0:not circle || result check_circle_1(head):\
%d\n", check_circle_2(head));
}
int main()
{
printf("-------testIS()--------\n");
testIS();
printf("-------testNO()--------\n");
testNO();
}
/****************************************
程序运行结果如下：
-------testIS()--------
1:is circle | 0:not circle || result check_circle_1(head):        1
1:is circle | 0:not circle || result check_circle_1(head):        1
-------testNO()--------
1:is circle | 0:not circle || result check_circle_1(head):        0
1:is circle | 0:not circle || result check_circle_1(head):        0

Process returned 0 (0x0)   execution time : 0.078 s
Press any key to continue.

*****************************************/

#include <cstdio>

typedef struct list

{

    int data;

    struct list *next;

}LIST;

/* Method 1: check the occurrence of p->next from head to p */

bool check_circle_1(LIST *head)

{

    LIST *p = head, *q= NULL;

    if (p == NULL)

        return false;

    while (p->next)

    {

        /* check whether p points to itself */

        if (p->next == p)

        {

            return true;

        }

        /* check the occurrence of p->next in head to p */

        q = head;

        while (q != p)

        {

            if (q == p->next)

            {

                return true;

            }

            q = q->next;

        }

        p = p->next;

    }

    /* p->next is NULL, not a circle */

    return false;

}

/* Method 2: q goes faster than p, if at last p == q, means there's circle */

/* 优点：逻辑上简单。 缺点：无法具体知道从哪个点拆开该圈 */

bool check_circle_2(LIST *head)

{

    LIST *p, *q;

    p = head;

    if (p == NULL)

        return false;

    q = p->next;

    while (p != NULL && q != NULL)

    {

        if (p == q)

        {

            return true;

        }

        p = p->next;

        if (q->next == NULL)

        {

            return 0;

        }

        else

        {

            q=q->next->next;

        }

    }

    return 0;

}

void testIS()

{

    LIST a, b, c, *head = &a;

    a.next = &b;

    b.next = &c;

    c.next = &a;

    printf("1:is circle | 0:not circle || result check_circle_1(head):\

        %d\n", check_circle_1(head));

    printf("1:is circle | 0:not circle || result check_circle_1(head):\

        %d\n", check_circle_2(head));

}

void testNO()

{

    LIST a, b, c, *head = &a;

    a.next = &b;

    b.next = &c;

    c.next = NULL;

    printf("1:is circle | 0:not circle || result check_circle_1(head):\

        %d\n", check_circle_1(head));

    printf("1:is circle | 0:not circle || result check_circle_1(head):\

        %d\n", check_circle_2(head));

}

int main()

{

    printf("-------testIS()--------\n");

    testIS();

    printf("-------testNO()--------\n");

    testNO();

}

/****************************************

程序运行结果如下：

-------testIS()--------

1:is circle | 0:not circle || result check_circle_1(head):        1

1:is circle | 0:not circle || result check_circle_1(head):        1

-------testNO()--------

1:is circle | 0:not circle || result check_circle_1(head):        0

1:is circle | 0:not circle || result check_circle_1(head):        0

Process returned 0 (0x0)   execution time : 0.078 s

Press any key to continue.

*****************************************/