Infix to Prefix Conversion in DSA C++

Program 1

// Program to Convert Infix to Prefix

#include<iostream>
#include<ctype.h>
#include<string.h>
#define clrscr() system("cls")
using namespace std;

char stack[50];   // stack
int top = -1; 

void push(char ch)
{
    stack[++top] = ch;
}

char pop()
{
    if(top == -1)
        return -1;
    else
        return stack[top--];
}

int priority(char ch)
{
    if(ch == '(')
        return 0;
    if(ch == '+' || ch == '-')
        return 1;
    if(ch == '*' || ch == '/')
        return 2;
    return 0;
}

// Function to reverse the string and swap '(' with ')'
void reverseExpr(char expr[]) 
{
    int len = strlen(expr);
    for(int i = 0; i < len / 2; i++)
        swap(expr[i], expr[len - i - 1]);

    for(int i = 0; i < len; i++)
    {
        if(expr[i] == '(')
            expr[i] = ')';
        else if(expr[i] == ')')
            expr[i] = '(';
    }
}

int main()
{
    char expr[50];   // expression
    char result[50];  //           [                ]
    int resIndex = 0;  
    char *p, x; 

    cout << "Enter the infix expression: ";
    cin >> expr;     //  ( A+B-C*D)     // (   D  *   C  -  B  +  A  )
                                                      //  0  1  2  3  4  5   6  7  8
    // Step 1: Reverse the expression and swap parentheses
    reverseExpr(expr);
    p = expr;
                                                // result[ D  C  *   B -  A  +   ]
    // Step 2: Apply infix to postfix logic on reversed expression
    while(*p != '\0')
    {
        if(isalnum(*p))
            result[resIndex++] = *p;    
        else if(*p == '(')
            push(*p);
        else if(*p == ')')
        {
            while((x = pop()) != '(')
                result[resIndex++] = x;
        }
        else
        {
            while(priority(stack[top]) >= priority(*p))
                result[resIndex++] = pop();
            push(*p);
        }
        p++;
    }

    while(top != -1)
        result[resIndex++] = pop();     

    result[resIndex] = '\0';

    // Step 3: Reverse the result to get prefix expression

    strrev(result);  // if not available, use manual reverse loop
    cout << "\nPrefix Expression: " << result << endl;
    // // result[ D  C  *   B -  A  +   ]
    //  +A-B*CD
    return 0;
}