Infix to Prefix , Using stack || Simple code

  // Write a program in c to convert In-Fix to pos-Fix
  #include <stdio.h>
  #include <math.h>
  #include <string.h>
  #include <stdlib.h>
  #define MAX 20
  void push(int);
  char pop();
  void infix_to_prefix();
  int precedence(char);
  char stack[20], infix[20], prefix[20];
  int top = -1;
  
  int main()
  {
    printf("\nINPUT THE INFIX EXPRESSION : \n");
    scanf("%s", infix);
    infix_to_prefix();
    return 0;
  }
  // method that pushes the elements onto the character stack
  void push(int pos)
  {
      if (top == MAX - 1)
      {
          printf("\nSTACK OVERFLOW\n");
      }
      else
      {
          top++;
          stack[top] = infix[pos];
      }
  }
  
  // method that removes character from stack and returns them
  char pop()
  {
    char ch;  
    if (top < 0)
    {
        printf("\nSTACK UNDERFLOW\n");
        exit(0);
    }
    else
    {
        ch = stack[top];
        stack[top] = '\0';
        top--;
        return (ch);
    }
    return 0;
  }
  // method that converts String from infix to prefix
  // all the strings are assumed to be valid expressions

  void infix_to_prefix()
  {
    int i = 0, j = 0;
    s  trrev(infix); // Reverse the infix expression
    while (infix[i] != '\0')
    {
        // if an alphabet is found then copy it to the output string
        if (infix[i] >= 'a' && infix[i] <= 'z')
        {
            prefix[j] = infix[i];
            j++;
            i++;
        }
        // In the reversed string, closing bracket will be found first so put it in the stack
        else if (infix[i] == ')' || infix[i] == '}' || infix[i] == ']')
        {
            push(i);
            i++;
        }
        // if an opening bracket is found, then
        else if (infix[i] == '(' || infix[i] == '{' || infix[i] == '[') /* when closing bracket is found */
        {
            if (infix[i] == '(')
            {
                while (stack[top] != ')') /* pop till opening bracket is found */
                {
                    prefix[j] = pop();
                    j++;
                }
                pop();
                i++;
            }
            else if (infix[i] == '[')
            {
                while (stack[top] != ']') /* pop till corresponding opening bracket is found */
                {
                    prefix[j] = pop();
                    j++;
                }
                pop();
                i++;
            }
            else if (infix[i] == '{')
            {
                while (stack[top] != '}') /*pop till corresponding opening bracket is found*/
                {
                    prefix[j] = pop();
                    j++;
                }
                pop();
                i++;
            }
        }
        else
        {
            // if the stack if empty, then we simply put the operator in stack
            if (top == -1)
            {
                push(i);
                i++;
            }
            // if the precedence of operator is less than the stack top then
            else if (precedence(infix[i]) < precedence(stack[top]))
            {
                prefix[j] = pop(); // pop the stack top and add it to the prefix string
                j++;
                // if you have an operator that has precedence greater than operator
                while (precedence(stack[top]) > precedence(infix[i]))
                {
                    prefix[j] = pop(); // Pop the operator
                    j++;
                    if (top < 0)
                    {
                        break;
                    }
                }
                push(i);
                i++;
            }
            // if the precedence of operator is greater than or equal to the stack top
            else if (precedence(infix[i]) >= precedence(stack[top]))
            {
                push(i); //  Push it onto the stack
                i++;
            }
        }
    }
    // At the end remove all the operators from the stack
    while (top != -1)
    {
        prefix[j] = pop();
        j++;
    }
    // Reverse the final string before output
    strrev(prefix);
    prefix[j] = '\0';
    printf("EQUIVALENT PREFIX NOTATION : %s ", prefix);
  }
  
  //Function to return precedence of operators 
  int precedence(char alpha)
  {
    if (alpha == '+' || alpha == '-')
    {
        return (1);
    }
    if (alpha == '*' || alpha == '/')
    {
        return (2);
    }
    return 0;
  }

When the above program is executed, it produces following results --<\b>
OUTPUT NO :- 01

  INPUT THE INFIX EXPRESSION : 
  (a+b+c)+d
  EQUIVALENT PREFIX NOTATION : +++abcd