查找鏈表中間節點

問題：鏈表長度未知，如何查找中間節點

分析：鏈表中間節點受鏈表長度影響，奇數長度如abc則中間節點爲b，偶數長度如abcd則中間節點爲bc。

解決方法：使用兩個指針ptr1 = ptr2 =head，同時遍歷鏈表，ptr1步長爲1，ptr2步長爲2.當ptr2到達鏈表尾時，ptr1即爲中間節點（如果鏈表長爲偶數，則ptr1->next也是中間節點）。

代碼如下：

#include<stdio.h>  
typedef struct node  
{  
    int data;  
    struct node *next;  
}NODE;  
  
NODE* create_list(int num)  
{  
  
    NODE *head = NULL;  
    NODE *cur = NULL;  
    int i = 0;  
      
    if (num <= 0)  
        return NULL;  
    while( i++ < num )  
    {  
        NODE *tmp = (NODE*)malloc(sizeof(NODE));  
        tmp->data = i;  
        tmp->next = NULL;  
          
        if ( i == 1)  
        {  
            head = tmp;  
        }  
        else  
        {  
            cur->next = tmp;  
        }  
        cur = tmp;  
    }  
    return head;  
}  
  
void print_list(NODE *head)  
{  
    printf("List:\n");  
    while(head != NULL)  
    {  
        printf("%d\t", head->data);  
        head = head->next;  
    }  
    printf("\n");  
}  
  
void search_middle_node(NODE *head)  
{  
    NODE *single_step = head;  
    NODE *double_step = head;  
    while(double_step != NULL)  
    {  
        if ( double_step->next == NULL )  
        {  
            printf("Middle Node :%d\n", single_step->data);  
            return;  
        }  
        if ( double_step->next->next == NULL)  
        {  
            printf("Middle Node :%d\t%d\n", single_step->data, single_step->next->data);  
            return;  
        }  
        single_step = single_step->next;  
        double_step = double_step->next->next;  
    }  
    return;  
}  
void main(){  
  
    NODE *head =  create_list(5);  
    print_list(head);  
    search_middle_node(head);  
}