ParseExpression.java

package fourEqualsTen;

import java.util.ArrayList;
import java.util.List;
import java.util.Stack;

public class ParseExpression {
    public static float evaluateExpression(String expression) {
        Stack<Float> values = new Stack<>();
        Stack<Character> operators = new Stack<>();

        for (int i = 0; i < expression.length(); i++) {
            char c = expression.charAt(i);

            if (Character.isDigit(c)) {
                StringBuilder num = new StringBuilder();
                while (i < expression.length() && (Character.isDigit(expression.charAt(i)) || expression.charAt(i) == '.')) {
                    num.append(expression.charAt(i));
                    i++;
                }
                i--;

                values.push(Float.parseFloat(num.toString()));
            } else if (c == '(') {
                operators.push(c);
            } else if (c == ')') {
                while (!operators.isEmpty() && operators.peek() != '(') {
                    values.push(applyOperator(operators.pop(), values.pop(), values.pop()));
                }
                operators.pop(); // Pop the '('
            } else if (isOperator(c)) {
                while (!operators.isEmpty() && precedence(c) <= precedence(operators.peek())) {
                    values.push(applyOperator(operators.pop(), values.pop(), values.pop()));
                }
                operators.push(c);
            }
        }

        while (!operators.isEmpty()) {
            values.push(applyOperator(operators.pop(), values.pop(), values.pop()));
        }

        return values.pop();
    }

    private static boolean isOperator(char c) {
        return c == '+' || c == '-' || c == '*' || c == '/';
    }

    private static int precedence(char operator) {
        switch (operator) {
            case '+':
            case '-':
                return 1;
            case '*':
            case '/':
                return 2;
        }
        return -1;
    }

    private static float applyOperator(char operator, float b, float a) {
        if (operator == '/' && b == 0) {
            return -1;
        }

        switch (operator) {
            case '+':
                return a + b;
            case '-':
                return a - b;
            case '*':
                return a * b;
            case '/':
                return a / b;
        }
        return 0;
    }

    public static List<String> evaluateExpressionResult(List<String> expressions)
    {
        List<String> res = new ArrayList<>();
        for (String expr: expressions)
        {
            float f = evaluateExpression(expr);
            if (f == 10)
            {
                res.add(expr);
            }
        }
        return res;
    }



}