棧（c實現)

棧作爲一種簡單但常用的數據結構，要想完全正確的實現還是需要經過一番考慮的。《劍指offer》這本書一再強調代碼的魯棒性。

下面是一個棧的簡單c語言實現，作爲一個學習過程記錄如下。

#include <stdio.h>
#include <stdlib.h>

int gError = 0; //用於棧爲空時，仍彈棧的標記
typedef struct Node {
int data;
struct Node* next;
} Node ;
int length(Node*);
void push(Node** stack,const int a)
{
        if(*stack==NULL)
        {
                *stack = (Node*)malloc(sizeof(Node));
                (*stack)->data = a;
                (*stack)->next = NULL;
        } else {
                Node* p = *stack;
                int len = length(p);
                for(int i=0;i<len-1;i++)
                {
                        p = p->next;
                }
                p->next = (Node*)malloc(sizeof(Node));
                p->next->data = a;
                p->next->next = NULL;
        }
}
int length(Node* head)
{
        Node* p = head;
        int count = 0;
        while(p!=NULL)
        {
                p = p->next;
                count++;
        }
        return count;
}
int pop(Node** head)
{
        if(*head==NULL)
        {
                fprintf(stdout,"null stack\n");
                gError = 1;
                return -1;
        }
        Node* p = *head;
        int len = length(p);
        int tmp = 0;
        Node* prev = NULL;
        if(len==1)
        {
                tmp = p->data;
                free(p);
                *head = NULL;
        } else {
                for(int i=0;i<len-1;i++)
                {
                       prev = p;
                       p = p->next;
                }
                prev->next = NULL;
                tmp = p->data;
                free(p);
        }
        return tmp;
}
void  print(Node* head)
{
        Node* p = head;
        while(p!=NULL)
        {
                fprintf(stdout,"%3d",p->data);
                p = p->next;
        }
}

int main(void)
{
        Node* head = NULL;
        push(&head,13);
        push(&head,14);
        push(&head,15);
        push(&head,16);
        print(head);
        printf("\n");
        printf("top is : %d\n",pop(&head));
        printf("top is : %d\n",pop(&head));
        printf("top is : %d\n",pop(&head));
        printf("top is : %d\n",pop(&head));
        return 0;
}