C program to check nesting of parentheses using a stack

#include <stdio.h>
#include <conio.h>
#include <string.h>
#define MAX 10
int top = –1;
int stk[MAX];
void push(char);
char pop();
void main()
{
char exp[MAX],temp;
int i, flag=1;
clrscr();
printf("Enter an expression : ");
gets(exp);
for(i=0;i<strlen(exp);i++)
{
 if(exp[i]=='(' || exp[i]=='{' || exp[i]=='[')
 push(exp[i]);
 if(exp[i]==’)’ || exp[i]==’}’ || exp[i]==’]’)
 if(top == –1)
flag=0;
 else
{
 temp=pop();
 if(exp[i]==')' && (temp=='{' || temp=='['))
flag=0;
 if(exp[i]=='}' && (temp=='(' || temp=='['))
flag=0;
 if(exp[i]==']' && (temp=='(' || temp=='{'))
flag=0;
 }
}
if(top>=0)
flag=0;
if(flag==1)
 printf("\n Valid expression");
else
 printf("\n Invalid expression");
}
void push(char c)
{
if(top == (MAX–1))
printf("Stack Overflow\n");
else
{
 top=top+1;
 stk[top] = c;
}
}
char pop()
{
if(top == –1)
printf("\n Stack Underflow");
else
 return(stk[top––]);
}