[Algorithm]一切始于ADT-表达式计算

Sedgewick的《算法》和Allen Weiss的《算法和数据结构》,这两本比较优秀的算法入门教材第一课都是ADT（abstract data type）.无疑，ADT非常有用且非常有趣的知识。刚好最近学校的C语言课程给了一道这样的题目..

1.问题描述

从标准输入中读入一个整数算术运算表达式，如5 - 1 * 2 * 3 + 12 / 2 / 2  = 。计算表达式结果，并输出。

要求：
1、表达式运算符只有+、-、*、/，表达式末尾的’=’字符表示表达式输入结束，表达式中可能会出现空格；
2、表达式中不含圆括号，不会出现错误的表达式；
3、出现除号/时，以整数相除进行运算，结果仍为整数，例如：5/3结果应为1。

暴力解也不是不可以，但如果使用栈的话，这道题就会变得简单一些..

2.思路

利用栈将中缀式转换成后缀表达式，然后再次利用栈对后缀式进行计算

3.实现

   #include <stdio.h>
   #include <string.h>
   #include <ctype.h>
   #include <stdlib.h>
   int length;
   struct ele
   {
       char op;
       int num;
       int end_flag;
   };
   int postfixtoresult(struct ele *t);
   void infix2postfix(struct ele *t);
   int ele_strlen(struct ele *t);
   void getrank(int *rank,char op);
   struct ele pop(struct ele *stack,struct ele **head);
   void push(struct ele **head,struct ele x);
   int isempty(struct ele *stack,struct ele **head);
   void to_ele_string(char *t,struct ele *ele_t);
   int main()
   {
       char s_buf[100];
       char clean_date[100];
       struct ele ele_buf[100];
       int l,i,k=0;
       for(i=0;i<100;i++)
           ele_buf[i].op=ele_buf[i].end_flag=ele_buf[i].num=0;
       fgets(s_buf,100,stdin);
       l=strlen(s_buf);
       for(i=0;i<l;i++)if(s_buf[i]==' '||s_buf[i]=='='||s_buf[i]=='
')continue;else clean_date[k++]=s_buf[i]; clean_date[k]='';
       to_ele_string(clean_date,ele_buf);
       infix2postfix(ele_buf);
       l=postfixtoresult(ele_buf);
       printf("%d",l);
       return 0;
               
   } 
   void to_ele_string(char *t,struct ele *ele_t)
   {
       char TEMP[10];
       int k=0,m=0,i=0;
       int l=strlen(t);
       for(i=0;i<l;i++)
       {
           if(isdigit(t[i]))
               TEMP[k++]=t[i];
           else
           {
               TEMP[k]='';
               ele_t[m].op = 0;
               ele_t[m].num = atoi(TEMP);
               ele_t[m].end_flag = 0;
               m++;
               ele_t[m].op = t[i];
               ele_t[m].num = 0;
               ele_t[m].end_flag = 0;
               m++;
               k=0;
               memset(TEMP,0,sizeof(TEMP));
           }
       }
       TEMP[k]='';
       ele_t[m].op = 0;
       ele_t[m].num = atoi(TEMP);
       ele_t[m].end_flag = 1;
   }
   int postfixtoresult(struct ele *t)
   {
       struct ele stack[100];
       struct ele *head=stack;
       struct ele TEMP;
       int l=ele_strlen(t);
       int i;
       for(i=0;i<l;i++)
       {
           if(t[i].op==0)
               push(&head,t[i]);
           else
           {
               int a,b,r=0;
               a=pop(stack,&head).num;
               b=pop(stack,&head).num;
               switch(t[i].op)
               {
                   case '+':r=a+b;break;
                   case '-':r=b-a;break;
                   case '*':r=a*b;break;
                   case '/':r=b/a;break;
               }
               TEMP.op=0;
               TEMP.num=r;
               TEMP.end_flag=0;
               push(&head,TEMP);
           }
       }
       return pop(stack,&head).num;
   }
   void infix2postfix(struct ele *t)
   {
       struct ele stack[100];
       struct ele *head=stack;
       struct ele out_buf[100];int k=0;
       int i;
       for(i=0;i<100;i++)
       {
           out_buf[i].op=0;
           out_buf[i].end_flag=0;
           out_buf[i].num=0;
       }
       int length=ele_strlen(t);
       int head_rank=0,p_rank=0;
       for(i=0;i<length;i++)
       {
           if(t[i].op==0)
               out_buf[k++].num=t[i].num;
           else
           {
               if(isempty(stack,&head))
               {
                   push(&head,t[i]);
               }
               else
               {
                   getrank(&p_rank,t[i].op);
                   getrank(&head_rank,(*(head-1)).op);
                   while(head_rank>=p_rank)
                   {
                       out_buf[k++]=pop(stack,&head);
                       if(isempty(stack,&head))break;
                       getrank(&head_rank,(*(head-1)).op);
                   }
                   push(&head,t[i]);
               }
           }
       }
       while(!isempty(stack,&head))
           out_buf[k++]=pop(stack,&head);
       out_buf[k-1].end_flag=1;
       memcpy(t,out_buf,100*sizeof(struct ele));
   }
   int ele_strlen(struct ele *t)
   {
       int i=1;
       while(!t++->end_flag)i++;
       return i;
   }
   void getrank(int *rank,char op)
   {
       switch(op)
       {
           case '*':*rank=2;break;
           case '/':*rank=2;break;
           case '+':*rank=1;break;
           case '-':*rank=1;break;
       }
   }
   int isempty(struct ele *stack,struct ele **head)
   {
       if(*head==stack)
           return 1;
       else return 0;
       return 1;
   }
   void push(struct ele **head,struct ele x)
   {
       **head=x;
       *head=*head+1;
   }
   struct ele pop(struct ele *stack,struct ele **head)
   {
       struct ele null;
       null.end_flag=1;
   
       if(isempty(stack,head))
           return null;
       else
       {
           *head=*head-1;
           return **head;
       }
       return null;
    }