Post Fix to Infix

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

// Stack type
struct Stack
{
    int top;
    unsigned capacity;
    int* array;
};

// Stack Operations
struct Stack* createStack(unsigned capacity)
{
    struct Stack* stack = (struct Stack*) malloc(sizeof(struct Stack));

    if (!stack)
        return NULL;

    stack->top = -1;
    stack->capacity = capacity;

    stack->array = (int*)malloc(stack->capacity * sizeof(int));

    if (!stack->array)
        return NULL;
    return stack;
}
int isEmpty(struct Stack* stack)
{
    return stack->top == -1;
}
char peek(struct Stack* stack)
{
    return stack->array[stack->top];
}
char pop(struct Stack* stack)
{
    if (!isEmpty(stack))
        return stack->array[stack->top--];
    return '$';
}
void push(struct Stack* stack, char op)
{
    stack->array[++stack->top] = op;
}


int isOperand(char ch)
{
    return (ch >= 'a' && ch <= 'z') || (ch >= 'A' && ch <= 'Z');
}

int prec(char ch)
{
    switch (ch)
    {
    case '+':
    case '-' :
        return 1;
    case '*':
    case '/':
        return 2;

    case '^':
        return 3;
    }
    return -1;
}

int infixToPostfix(char* exp)
{

    int k , i ;
    struct Stack* stack = createStack(strlen(exp));
    if (!stack)
        return -1;
    for ( i = 0, k = -1; exp[i]; ++i)
    {
        //if scanned character is an operand , add to the output
        if (isOperand(exp[i]))
            exp[++k] = exp[i];
        // If the scanned character is an ‘(‘, push it to the stack.
        else if (exp[i] == '(')
            push(stack, exp[i]);
        //  If the scanned character is an ‘)’, pop and output from the stack
        // until an ‘(‘ is encountered.
        else if (exp[i] == ')')
        {
            while (!isEmpty(stack) && peek(stack) != '(')
                exp[++k] = pop(stack);
            if (!isEmpty(stack) && peek(stack) != '(')
                return -1; //invalid expression
            else
                pop(stack); // pop starting braces (
        }

        else //an operator is encountered
        {
            while (!isEmpty(stack) && prec(exp[i]) <= prec(peek(stack)))
                exp[++k] = pop(stack);
            push(stack, exp[i]);
        }

    }
    // pop all the operators from the stack
    while (!isEmpty(stack))
        exp[++k] = pop(stack);

    exp[++k] = '\0';
    printf("%s\n", exp);
}
// Driver program to test above functions
int main()
{
    char exp[] = "a+b*(c^d-e)^(f+g*h)-i";
    infixToPostfix(exp);
    getchar();
    return 0;
}