InFix to PostFix and PreFix conversion [C++]

To Convert from InFix to Postfix:

1. Print out constants from InFix directly onto the Postfix output

2. For operators, insert them into a stack

   • Before insertion, make sure to pop out (and print to Postfix output) all operator that have higher precedence than that of the operator currently being inserted.

3. After the entire Infix is processed, flush the stack (if it is not empty) onto the Postfix output

Note: In case of InFix to Prefix, we must start processing the string from the RIGHT most to LEFT most instead of the usual LEFT to RIGHT.

This code can handle only numbers from 0-9 (single digit) and +, -, *, / operators.

//Evaluate the given expression
#include<iostream>
#include<stack>
#include<vector>
using namespace std;

//Only single digit numbers permitted

int GetPrecedence(char op)
{
    switch (op)
    {
    case '+':
    case '-':
        return 1;
    case '*':
    case '/':
    default:
        return 2;
    }
}

char* ConvertInorderToPreOrder(char* InOrderExp, int Size)
{
    stack<char> Operators;
    vector<char> PostOrderResult;
    for (int i = Size-1; i >=0; i--)
        if (InOrderExp[i] >= '0'&& InOrderExp[i] <= '9')
            PostOrderResult.push_back(InOrderExp[i]);
        else
        {
            while (!Operators.empty() && GetPrecedence(Operators.top()) >= GetPrecedence(InOrderExp[i]))
            {
                PostOrderResult.push_back(Operators.top());
                Operators.pop();
            }
            Operators.push(InOrderExp[i]);
        }
    while (!Operators.empty())
    {
        PostOrderResult.push_back(Operators.top());
        Operators.pop();
    }
    char *ResultString = (char*)malloc(sizeof(char)*PostOrderResult.size()+1);
    memset(ResultString, 0, sizeof(char)*(PostOrderResult.size() + 1));
    for (int i = 0; i < PostOrderResult.size(); i++)
        ResultString[i] = PostOrderResult[i];
    ResultString[PostOrderResult.size()] = '\0';
    _strrev(ResultString);
    return ResultString;
}

char* ConvertInorderToPostOrder(char* InOrderExp, int Size)
{
    stack<char> Operators;
    vector<char> PostOrderResult;
    for (int i = 0; i < Size; i++)
        if (InOrderExp[i] >= '0'&& InOrderExp[i] <= '9')
            PostOrderResult.push_back(InOrderExp[i]);
        else
        {
            while (!Operators.empty() && GetPrecedence(Operators.top()) >= GetPrecedence(InOrderExp[i]))
            {
                PostOrderResult.push_back(Operators.top());
                Operators.pop();
            }
            Operators.push(InOrderExp[i]);
        }
    while (!Operators.empty())
    {
        PostOrderResult.push_back(Operators.top());
        Operators.pop();
    }
    char *ResultString = (char*)malloc(sizeof(char)*PostOrderResult.size()+1);
    memset(ResultString, 0, sizeof(char)*(PostOrderResult.size()+1));
    for (int i = 0; i < PostOrderResult.size(); i++)
        ResultString[i] = PostOrderResult[i];
    ResultString[PostOrderResult.size()] = '\0';
    return ResultString;
}

int main()
{
    char InExp[] = "2+3*7-5*2-4/2*5+1-4+2/2+1";
    cout << "\nInput: " << InExp;
    cout << "\n\n";
    cout << "\nPostFix = " << ConvertInorderToPostOrder(InExp, strlen(InExp));
    cout << "\nPreFix = " << ConvertInorderToPreOrder(InExp, strlen(InExp));
    cout << "\n\nEnd of code\n\n";
    system("pause");
    return 0;
}