Java RPN (reverse polish notation) infix for postfix

Question

I am sure that the stacks are used to build PRN and '(' are ignored, but it seems that this is not so. For example:

• input 1: 52+ (1 + 2) * 4-3
• input 2: 52 + ((1 + 2) * 4) -3
• input 3: (52 + 1 + 2) * 4-3

Input 1 and input 2 must be the same, and input 1 and input 3 must be different.

• output 1: 52 1 2 + 4 3 - * +
• pin 2: 52 1 2 + 4 * 3 - +
• pin 3: 52 1 2 + 4 3 - * +

    public static String Infix2(String input) {
        char[] in = input.toCharArray();
        Stack<Character> stack = new Stack<Character>();
        StringBuilder out = new StringBuilder();

        for (int i = 0; i < in.length; i++)
            switch (in[i]) {
            case '+':
            case '*':
            case '-':
                out.append(' ');
                stack.push(in[i]);
                break;
            case ' ':
            case '(':
                break;
            case ')':
                out.append(' ');
                out.append(stack.pop());
                break;
            default:
                out.append(in[i]);
                break;
            }

        while (!stack.isEmpty()) {
            out.append(' ');
            out.append(stack.pop());
        }

        return out.toString();
    }

Assuming I want input 1 and 3 to work as well, which approach should I use?

edit: After the changes, "+", "-", "*" and "/" worked for the given inputs.

public static String Infix2(String input) {
    if (input == null)
        return "";
    char[] in = input.toCharArray();
    Stack<Character> stack = new Stack<Character>();
    StringBuilder out = new StringBuilder();

    for (int i = 0; i < in.length; i++)
        switch (in[i]) {
        case '+':
        case '-':
            while (!stack.empty()
                    && (stack.peek() == '*' || stack.peek() == '/'))
                out.append(' ').append(stack.pop());
        case '*':
        case '/':
            out.append(' ');
        case '(':
            stack.push(in[i]);
        case ' ':
            break;
        case ')':
            while (!stack.empty() && stack.peek() != '(')
                out.append(' ').append(stack.pop());
            if (!stack.empty())
                stack.pop();
            break;
        default:
            out.append(in[i]);
            break;
        }

    while (!stack.isEmpty())
        out.append(' ').append(stack.pop());

    return out.toString();
}

Answer 1

( ). , + - . :

• , ,

, 52+ (1 + 2) * 4-3, :

 52+          => +
 52+(         => + (
 52+(1+       => + ( + 
 52+(1+2)     => +       //right parentheses popped +
 52+(1+2)*4   => + * 
 52+(1+2)*4-3 => + -     //can't put - on top of *, so pop off *
 ... and then pop the stack until it empty.

( , ) . -.

for (int i = 0; i < in.length; i++)
        switch (in[i]) {
        case '+':
        case '-':
            while (!stack.empty() && (stack.peek() == '*' || stack.peek() == '/')) {
                out.append(' ');
                out.append(stack.pop());
            }
            out.append(' ');
            stack.push(in[i]);
            break;
        case '*':
        case '/':
            out.append(' ');
            stack.push(in[i]);
            break;
        case '(':
            stack.push(in[i]);
            break;
        case ')':
            while (!stack.empty() && stack.peek() != '(') {
                out.append(' ');
                out.append(stack.pop());
            }
            stack.pop();
            break;
        default:
            out.append(in[i]);
            break;
        }

Answer 2

, , : - (TDD). : - , JUnit - infix2, () , infix2 .

,

assertequals("1", "1"); // positive number
assertequals("-1", "-1"); // negative number
assertequals("1+1", "1 1 +"); // simple addition
assertequals(" 1 + 1 ", "1 1 +"); // simple addition with whitechars
assertequals(" 1 + +1 ", "1 -1 +"); // simple addition with pos. number & whitechars
assertequals(" 1 + -1 ", "1 -1 +"); // simple addition with neg. number & whitechars
assertequals("(1+1)", "1 1 +"); // simple addition with brackets

,

String[] illegalExpressions = {null, "", " ", "1 +", "1 + 1)"};

assertequals("52+(1+2)*4-3", "52 1 2 + 4 * 3 -");
assertequals("52+((1+2)*4)-3", "52 1 2 + 4 * 3 -");
assertequals("(52+1+2)*4-3", "52 1 + 2 + 4 * 3 -");