/* Program to check nesting of parentheses using stack */
#include<stdio.h>#define MAX 20
#define true 1
#define false 0
int top = -1;
int stack[MAX];
push(char);
char pop();
main()
{
char exp[MAX],temp;
int i,valid=true;
printf("Enter an algebraic 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) /* stack empty */
valid=false;
else
{
temp=pop();
if( exp[i]==')' && (temp=='{' || temp=='[') )
valid=false;
if( exp[i]=='}' && (temp=='(' || temp=='[') )
valid=false;
if( exp[i]==']' && (temp=='(' || temp=='{') )
valid=false;
}/*End of else */
}/*End of for*/
if(top>=0) /*stack not empty*/
valid=false;
if( valid==true )
printf("Valid expression\n");
else
printf("Invalid expression\n");
}/*End of main()*/
push(char item)
{
if(top == (MAX-1))
printf("Stack Overflow\n");
else
{
top=top+1;
stack[top] = item;
}
}/*End of push()*/
char pop()
{
if(top == -1)
printf("Stack Underflow\n");
else
return(stack[top--]);
}
