将中缀表达式转换为后缀

Question

这是我写的代码。我没有得到正确的输出，有时我在操作数之间得到随机符号。我哪里出错了？

#include<stdio.h>
#include<ctype.h>
#include<string.h>
#define size 20
struct postfix    //Postfix Expression Stack
{
 char post[size];
 int posttop;
};
struct symbol   //Symbol Stack
{
 char sym[size];
 int symtop;
};

int priority(char ch)   //Checks the priority of Operatoes
{
 switch(ch)
 {
      case '+':
      case '-':
           return 1;
      case '*':
      case '/':
           return 2;
      case '^':
           return 3;
 }
}

 void push_post(struct postfix *s, char c)  //pushes character to postfix stack
 {
 s->posttop++;
 s->post[s->posttop]=c;
 }

  void push_sym(struct symbol *s, char c) //pushes character to postfix expression stack
  {
   s->symtop++;
   s->sym[s->symtop]=c;
   }

  char pop(struct symbol *s)
  {
   return(s->sym[(s->symtop)--]);  //returns the popped character
   }

  void show( struct postfix p)
  {
   int i;
   for(i=0;i<=p.posttop;i++)
      printf("%c",p.post[i]);
   }

 void main()
 {
 struct postfix p;
 struct symbol s;
 p.posttop=-1;
 s.symtop=-1;
 char ch;
 char exp[20];
 int i;
 printf("Enter your infix expression : ");
 gets(exp);
 int l=strlen(exp);
 for(i=0;i<=l;i++)
 {
      if (isalpha(exp[i])){
          push_post(&p,exp[i]);
      }

      else if(exp[i]=='(')
                push_sym(&s,exp[i]);

      else if (exp[i]==')')
      {
           while(s.sym[s.symtop]!='(')
           {
                ch = pop(&s);
                push_post(&p,ch);
           }
           ch=pop(&s); //pop the opening bracket and do nothing
      }

      else
      {
           while(priority(s.sym[s.symtop])<=priority(exp[i])) //while priority of top operator of stack is less than the read operator
           {
                ch=pop(&s); // Pop the top operator of symbol stack
                push_post(&p,ch); //push the operator popped from symbol stack to prefix stack
           }
           push_sym(&s,exp[i]); //push the read symbol
      }
 }
 while(s.symtop!=-1)
 {
      ch=pop(&s);
      push_post(&p, ch);
 }
 printf("Postfix Expression : ");
 show(p);
 getch();

 }

导致此错误的原因是什么？它适用于像a+b+c等小型表达式。但是我的大型表达式和大括号都会出错。