Infix to postfix conversion

/*logic is (AT GFG) that in the stack lower priority operator cannot be put above higher priority operator and Ravi sir's algo is different 
in that when + is compared with ( ,+ will have lower priortiy and when ( is compared with + then ( will have lower priority */ 

#include <stdio.h>
#include <string.h>
#include <math.h>
#include <stdlib.h>
#include <limits.h>
struct node{
    int data;
    struct node *link;
};
struct node *head=NULL;
void push(char x)
{
     
    struct node* p=(struct node*) malloc (sizeof(struct node));
    if(p==NULL)
        {
        printf("overflow");
    return;
    }
    p->data=x;
  //  p->link=NULL;
    p->link=head;
    head=p;                //here head acts like top
    
}
char pop()
    {
    char item;
    struct node *p;
   
    if(head==NULL){
        //printf("underflow");
        return -1;
    }
    item=head->data;
    p=head;
    head=head->link;
    free(p);
    return item;
}
int match(char x)
    {
   
   switch (x)
    {
    case '+':
    case '-':
        return 1;
 
    case '*':
    case '/':
        return 2;
 
    case '^':
        return 3;
    }
    return -1;

}
void check()
    {
   
       char value,arr[10000];
          scanf("%s",arr);
       int i=0;  
     while(arr[i])
         {
       value=arr[i]; 
   if((value>='a'&&value<='z')||(value>='A'&&value<='Z'))
     printf("%c",value);
         
         else if(value=='(')
             push(value);
  else if(value==')'){
    //printf("P%c \n",value);
      while(head!=NULL&&head->data!='(')
              printf("%c",pop());
      if (head!=NULL&&head->data!='(')
                printf("invalid"); // invalid expression                
            else
                pop();
      
   }
    else 
        {
       while(head!=NULL&&(match(head->data)>=match(value)))
            printf("%c",pop());
       
           push(value);
         
       
    }  
    
    i++;    
     }  
    while(head!=NULL)
   printf("%c",pop());
}
int main()
    {
    int x,T,t=0,n,i;
    scanf("%d",&n);
   
    while(n--){
       (check());
          printf("\n");
          
    }
            return 0;
}