Why wrong answer for Transform the Expression problem?

Here is my code for Transform the Expression problem :-

#include<stdio.h>
#include<stdlib.h>

int prior(char ch)
{
	if(ch == '+')
	{
		return 1;
	}
	else if(ch == '-')
	{
		return 2;
	}
	else if(ch == '*')
	{
		return 3;
	}
	else if(ch == '/')
	{
		return 4;
	}
	else if(ch == '^')
	{
		return 5;
	}
	else if(ch == '(')
	{
		return 0;
	}
	
}

void push(char *stk,int *top,char ch )
{
	if(*top < 399)
	{
		stk[++(*top)] = ch;
	}
}

char pop(char *stk,int *top)
{
	if((*top)>=0)
	{
		char temp;

		temp = stk[(*top)--];
		return temp;

	}
}

int main()
{
	int n,stray;

	scanf("%d",&n);

	char *op = (char *)calloc(400,sizeof(char));
	char *ex = (char *)calloc(400,sizeof(char));
	char *string = (char *)calloc(400,sizeof(char));

	int i,t_op ,t_ex;
	char ch;

	for(i=0;i<n;i++)
	{
		scanf("%s",string);
	
		t_op = -1;
		t_ex = -1;

		while(*string != '\0')
		{
			if((*string>=65 && *string<=90) || (*string>=97 && *string<=122))
			{
				push(ex,&t_ex,*string);
			}
			else 
			{
				
				if(*string == '(')
				{
					push(op,&t_op,*string);
				}
				else if(*string == ')')
				{
					while((ch = pop(op,&t_op)) != '(')
					{
						push(ex,&t_ex,ch);
					}
				}
				else if(prior(*string) > prior(op[t_op]))
				{
					push(op,&t_op,*string);
				}
				else
				{
					while(prior(*string) < prior(op[t_op]))
					{
						push(ex,&t_ex,pop(op,&t_op));
					}
					
					push(op,&t_op,*string);

				}

				
			}

			string++;
		}

		printf("%s\n",ex);

	}

	return 0;
}

Can anyone tell me why it is giving wrong answer and is there any link from where we can find the test cases for a given problem?

//