Infix to Postfix.c

#include <limits.h>  
#include <stdio.h>  
#include <stdlib.h>  
#define MAX 20  
  
char stack[MAX];
char expression[10000];  
int top = -1; 

  
int isEmpty()  
{  
    return top == -1;  
}  
int isFull()  
{  
    return top == MAX - 1;  
}  
  
char peek()  
{  
    return stack[top];  
}  
  
char pop()  
{  
    if(isEmpty())  
        return -1;  
  
    char ch = stack[top];  
    top--;  
    return(ch);  
}  
  
void push(char oper)  
{  
    if(isFull())  
        printf("Stack Full!!!!");  
     
    else{  
        top++;  
        stack[top] = oper;  
    }  
}  
  
int checkIfOperand(char ch)   
{   
    return (ch >= 'a' && ch <= 'z') || (ch >= 'A' && ch <= 'Z');   
}   
  
int precedence(char ch)   
{   
    switch (ch)   
    {   
    case '+':   
    case '-':   
        return 1;   
  
    case '*':   
    case '/':   
        return 2;   
  
    case '^':   
        return 3;   
    }   
    return -1;   
}   
  
int covertInfixToPostfix(char* expression)   
{   
    int i, j;  
  
    for (i = 0, j = -1; expression[i]; ++i)   
    {   
       if (checkIfOperand(expression[i]))   
            expression[++j] = expression[i];   
  
        else if (expression[i] == '(')   
            push(expression[i]);   
  
        else if (expression[i] == ')')   
        {   
            while (!isEmpty() && peek() != '(')   
                expression[++j] = pop();   
            if (!isEmpty() && peek() != '(')   
                return -1;             
            else  
                pop();   
        }  
        else   
        {   
            while (!isEmpty() && precedence(expression[i]) <= precedence(peek()))   
                expression[++j] = pop();   
            push(expression[i]);   
        }   
  
    }   
  
    while (!isEmpty())   
        expression[++j] = pop();   
  
    expression[++j] = '\0';   
    printf( "%s", expression);   
}   
  
int main()  
{  
   printf("Enter an infix expression");
    scanf("%s", &expression);
//char expression[] = "((A+B)*D)^(E-F)";   
    
    covertInfixToPostfix(expression);   
    return 0;   
}