Runtime error SIGSEGV infix to postfix - c++

Question

This code works fine on my machine but when i upload it to codechef it gives me a runtime error SIGSEGV. Can anyone please point out the error in my code? This is the question i made it for http://www.codechef.com/problems/ONP/
#include<iostream>
#include<string>
using namespace std;
class stack
{
public:
void push(char a)
{
++top;
arr[top]=a;
}
void pop()
{
top--;
}
void initialize(int size)
{
top=-1;
max=size;
}
bool chckfull()
{
return (top==max-1);
}
bool chckempty()
{
return (top==-1);
}
char front()
{
return arr[top];
}
private:
int top;
int max;
char arr[404];
};
int chckalphanum(char y)
{
if((y>='a')&&(y<='z'))
return 1;
else if ((y>='A')&&(y<'Z'))
return 1;
else if((y>='0')&&(y<='9'))
return 1;
return 0;
}
int pre(char x)
{
if(chckalphanum(x))
return 0;
if(x=='(')
return -1;
else if(x=='^')
return 3;
else if((x=='/')||(x=='*'))
return 2;
else
return 1;
}
int main ()
{
std::ios::sync_with_stdio(false);
string s, s1=")";
char q[404];
int qmax=0,t;
stack prs;
scanf("%d", &t);
while(t--)
{
cin>>s;
prs.initialize(s.length());
prs.push('(');
s=s+s1;
for(int i=0; i<s.length(); i++)
{
if(s[i]=='(')
prs.push('(');
else if(chckalphanum(s[i]))
{
q[qmax]=s[i];
qmax++;
}
else if(s[i]==')')
{
while(prs.front()!='(')
{
q[qmax]=prs.front();
qmax++;
prs.pop();
}
prs.pop();
}
else
{
while(pre(prs.front())>=pre(s[i]))
{
q[qmax]=prs.front();
qmax++;
prs.pop();
}
prs.push(s[i]);
}
}
for(int i=0; i<qmax; i++)
cout<<q[i];
cout<<"\n";
qmax=0;
}
return 0;
}

Answer 1

I just commented out the below line from your solution and it got accepted in codechef.
std::ios::sync_with_stdio(false);
I am not sure if you are aware of what the above line does but I will try to explain to the best of my knowledge. Better answers will definitely follow in due course from the community.
"With stdio synchronization turned off, iostream standard stream objects may operate independently of the standard C streams (although they are not required to), and mixing operations may result in unexpectedly interleaved characters."
Quoting from cppreference.
"Concurrent access to the same stream object may cause data races."
Since you have turned off the synchronization between stdio (C style I/O) and iostream (C++ style I/O)
and you continued using scanf and cin simultaneously interleaved, I suspect you got a runtime error.
For more research, please go through : http://www.cplusplus.com/reference/ios/ios_base/sync_with_stdio/
Hope it clarifies a bit, if not fully. Thanks!