#include <stdio.h>
#include <string.h>
#include <ctype.h>

struct stack
{
	char data[100];
	int top;
};

void push(struct stack *s, char x)
{
	s->data[++s->top]=x;
}

char pop(struct stack *s)
{
	return s->data[s->top--];
}

int rank(char x)
{
	if(x=='+' || x=='-'){return 0;}
	if(x=='*' || x=='/'){return 1;}
}

void convert(char c[])
{
	char x[100];
	struct stack s;
	int i, j, xlen;
	s.top=-1;
	xlen=-1;
	for(i=0; i<=strlen(c); i++)
	{
		if(i==strlen(c))
		{
			for(j=s.top; j>=0; j--)
			{
				char tmp;
				tmp=pop(&s);
				if(tmp!=')') x[++xlen]=tmp;
			}
			break;
		}
		if(isdigit(c[i])!=0)
		{
			push(&s, c[i]);
			continue;
		}
		if(c[i]=='+' || c[i]=='-' || c[i]=='*' || c[i]=='/')
		{
			if(rank(c[i])>=rank(s.data[s.top]) || rank(s.data[s.top])=='(')
			{
				push(&s, c[i]);
				continue;
			}
			if(rank(c[i])<rank(s.data[s.top]))
			{
				while(rank(c[i])<rank(s.data[s.top]) && s.data[s.top]!='(')
				{
					x[++xlen]=pop(&s);
					if(s.top==-1) break;
				}
				push(&s, c[i]);
				continue;
			}
		}
		if(c[i]=='(')
		{
			push(&s, c[i]);
			continue;
		}
		if(c[i]==')')
		{
			while(s.data[s.top]!='(')
			{
				x[++xlen]=pop(&s);
			}
			s.top--;
			continue;
		}
	}
	printf("This is the postfix form: ");
	for(i=0;i<=strlen(x);i++)
	{
		if(isdigit(x[i])!=0)
		{printf("%c",x[i]);}
		if(x[i]=='+' || x[i]=='-' || x[i]=='*' || x[i]=='/')
		{printf("%c",x[i]);}
	}
	printf("\n");
}

void main()
{
	int choice,loop=1;
	char exp[100];
	while(loop!=0)
	{
		printf("Please input an infix expression: ");
		scanf("%s", &exp);
		convert(exp);
		loop=2;
		while(loop==2)
		{
			printf("Do you want to exit from program? (0 = Yes || 1 = No)\n");
			printf("Choice: ");
			scanf("%d",&choice);
			switch(choice)
			{
			case 0:loop=0;break;
			case 1:loop=1;break;
			default:printf("Invalid choice!\n");
			}
		}
	}
}