can anyone find why is this program to solve postfix expression giving runtime error

[jinnejeevansai]

#include<stdio. h>
#include<stdlib .h>
struct stack{
int top;
int n[100];
}s;
void push(int a)
{
s.n[s.top+1]=a;
s.top++;
}
void pop(char a)
{
int c,b;
b=s.n[s.top];
c=s.n[s.top-1];
s.top--;
switch(a)
{
case '+':
s.n[s.top]=b+c;
break;
case '-':
s.n[s.top]=b-c;
break;
case '*':
s.n[s.top]=b*c;
break;
case '/':
s.n[s.top]=b/c;
break;
}

}
int main()
{
s.top=-1;
int m,i,k;
char a[100],c[100];
scanf("%d",&m);
for(i=0;i<m;i++ )
{ int j=0;

while(1)
{

scanf("%c",&a[j]);
if(a[j]=='?')
break;
else if(a[j]==' ')
{
push(atoi(a));
j=0;
}
else if(a[j]=='+'||'-'||'*'||'/')
pop(a[j]);
else
j++;
}
printf("%d",s.n[s.top]);

}

}

[weaknessforcats]

This sort of thing is common in software development. It is a signal for you to step through your code using your debugger. If you don't know how to use it, then this is a great opportunity to learn how.

All I could do is do your debugging for you but this would be a disservice since I would rob you of a learning opportunity.