利用线性表和栈实现 中缀表达式转后缀表达式 并进行计算

中缀表达式转后缀表达式

public class InfixToSuffix {
    public static void main(String[] args) {
        String expression = "(10+20/2*3)/2+8";
        expression = infixToSuffix(expression);
        System.out.println(expression);
    }

    public static String infixToSuffix(String expression) {
        //操作符的栈
        ArrayStack<String> opStack = new ArrayStack<>();
        //后缀表达式的线性表
        ArrayList<String> suffixList = new ArrayList<>();

        //格式化字符串
        expression = insertBlanks(expression);
        String[] tokens = expression.split(" ");
        for (String token : tokens){
            //过滤空串
            if (token.length() == 0){
                continue;
            }
            if (isOperator(token)){
                /*
                什么时候操作符进栈
                1.栈为空
                2.栈顶是（
                3.栈顶是操作符，且优先级比当前token小

                什么时候需要将栈顶操作符出栈？
                1.栈顶操作符的优先级 >= 当前token
                */
                while(true){
                    if (opStack.isEmpty() || opStack.peek().equals("(") || priority(opStack.peek()) < priority(token)){
                        opStack.push(token);
                        break;
                    }
                    suffixList.add(opStack.pop());
                }
            }else if (token.equals("(")){
                opStack.push(token);
            }else if (token.equals(")")){
                while (!opStack.peek().equals("(")){
                    suffixList.add(opStack.pop());
                }
                opStack.pop();
            }else if (isNumber(token)){
                suffixList.add(token);
            }else {
                throw new IllegalArgumentException("wrong char:" + expression);
            }
        }
        while (!opStack.isEmpty()){
            suffixList.add(opStack.pop());
        }
        StringBuilder sb = new StringBuilder();
        for (int i = 0; i < suffixList.size(); i++) {
            sb.append(suffixList.get(i));
            sb.append(' ');
        }
        return sb.toString();
    }

    private static int priority(String token) {
        if (token.equals("+") || token.equals("-")){
            return 0;
        }
        if (token.equals("*") || token.equals("/")){
            return 1;
        }
        return -1;
    }

    private static boolean isNumber(String token) {
        return token.matches("\\d+");
    }

    private static boolean isOperator(String token) {
        return token.equals("+") || token.equals("-") || token.equals("*") || token.equals("/");
    }

    //对原表达式进行格式化处理 给所有的非数字字符左右添加空格
    private static String insertBlanks(String expression) {
        StringBuilder sb = new StringBuilder();
        for (int i = 0; i < expression.length(); i++) {
            char c = expression.charAt(i);
            if (c == '(' || c ==')' || c == '+' || c =='-' || c == '*' || c == '/'){
                sb.append(' ');
                sb.append(c);
                sb.append(' ');
            }else {
                sb.append(c);
            }
        }
        return sb.toString();
    }
}

对于后缀表达式，我们利用栈进行计算，遍历后缀表达式，遇到数字我们直接进栈，碰到+ - * /我们提取两个数字进行计算后直接入栈

public class SuffixCalculator {
    public static void main(String[] args) {
        String infixExpression = "(10+20/2*3)/2+8";
        String suffixExpression = InfixToSuffix.infixToSuffix(infixExpression);
        int result = evaluateSuffix(suffixExpression);
        System.out.println(result);
    }

    private static int evaluateSuffix(String expression) {

        ArrayStack<Integer> stack = new ArrayStack<>();
        String[] tokens = expression.split(" ");
        for (String token: tokens) {
            if (token.length() == 0 ){
                continue;
            }
            if (isNumber(token)){
                stack.push(new Integer(token));
            }else {
                processAnOperator(stack,token);
            }
        }
        return stack.pop();
    }

    private static boolean isNumber(String token) {
        return token.matches("\\d+");
    }

    private static void processAnOperator(ArrayStack<Integer> stack, String token) {

        int num1 = stack.pop();
        int num2 = stack.pop();
        if (token.equals("+")){
            stack.push(num2 + num1);
        }else if (token.equals("*")){
            stack.push(num2 * num1);
        }else if (token.equals("-")){
            stack.push(num2 - num1);
        }else if (token.equals("/")){
            stack.push(num2 / num1);
        }
    }

}