String Recognition

	/* String Recognition */

	#include<stdio.h>
	#include<conio.h>
	#include<string.h>
	#define SIZE_OF_STACK 50

	char alphabet[50]="";
	char input_string[50]="";
	int no_of_alphabets=0,i;

	struct stack
	{
		char slot[SIZE_OF_STACK];
		int TOP;
	};

	int push(st,item)
	struct stack *st;
	char item;
	{
		if(st->TOP==SIZE_OF_STACK-1)
		{
			return 0; /* Push Operation Unsuccessful (Stack Overflow) */
		}
		else
		{
			st->TOP++;
			st->slot[st->TOP]=item;
			return 1; /* Push Operation Successful */
		}
	}

	char pop(st)
	struct stack *st;
	{
		if(st->TOP==-1)
		{
			return -1; /* Stack Underflow */
		}
		else
		{
			(st->TOP)--;
			return st->slot[st->TOP+1];
		}
	}

	int check_alphabet(c)
	char c;
	{
		int temp;
		for(temp=0;temp<=i;temp++)
		{
			if(c==alphabet[temp])
			{
				return 0; /* Character Repeated */
			}
		}

		return 1; /* Character Okay */
	}

	char next_char(string)
	char string[50];
	{
		static int count=0;
		count++;
		return string[count-1];
	}

	main()
	{
	 char temp,next;
	 struct stack s;
	 int x;
	 s.TOP=-1;
	 clrscr();
	 printf("\nHow many alphabets does the language have ? : ");
	 scanf("%d",&no_of_alphabets);

	 for(i=0;i<no_of_alphabets;i++)
	 {
		printf("\nAlphabet %d : ",i+1);
		temp=getche();
		if(check_alphabet(temp)==1)
		{
			alphabet[i]=temp;
		}
		else
		{
			printf("	ERROR : Character already exists !");
			i--;
		}
	 }
	 alphabet[i]='\0';
	 printf("\n\nEnter Input String : ");
	 scanf("%s",input_string);

	 push(&s,alphabet[i-1]);
	 next=next_char(input_string);
	 while(next!=alphabet[i-1])
	 {
		if(next=='\0')
		{
			printf("\nInvalid String !");
			goto exit;
		}
		else
		{
			push(&s,next);
		}
		next=next_char(input_string);
	 }

	 while(s.slot[s.TOP]!=alphabet[i-1])
	 {
		next=next_char(input_string);
		temp=pop(&s);
		if(next!=temp)
		{
			printf("\nInvalid String !");
			goto exit;
		}
	 }
	 next=next_char(input_string);

	 if(next=='\0')
	 {
		printf("\nValid String !");
	 }
	 else
	 {
		printf("\nInvalid String !");
	 }
	exit:
	getch();
	}