java 将中缀表达式转换成后缀表达式(逆波兰表达式)

1.该方法仅仅支持常数(大于等于零的整数),不支持数字前带有负号。

2.字符与字符之间应用空格符号隔开

import java.util.Stack;

/**
 * 波兰表达式转换
 * @author Haidnor
 * @creat 2020-03-14-14:21
 */
public class PolishNotationFactory {
    /**
     * 将运算式字符串转换成逆波兰表达式 (toReversePolishNotation)
     * 例如: ( ( 1 + 2 ) * 1 ) * ( 1 + 2 ) - ( 3 / 1 )
     * 返回: 1 2 + 1 * 1 2 + * 3 1 / -
     * @param expression 运算式
     * @return ReversePolishNotation
     */
    public String toRPN(String expression) {
        String[] split = expression.split(" ");
        StringBuffer sb = new StringBuffer();
        StringBuffer buffer = new StringBuffer();
        Stack<String> operators = new Stack<String>();
        Boolean isBracket  = false;
        int bracket = 0;
        int i = 0;
        for (String s : split) {
            if (s.equals("(")) {
                isBracket = true;
                bracket++;
                i++;
                continue;
            } else if (s.equals(")")) {
                isBracket = false;
                bracket--;
            }
            if (isBracket == false & buffer.length() == 0 & bracket == 0) {
                if (this.isOperator(s)) {
                    operators.push(s);
                    continue;
                }
            }
            if (isBracket & bracket >= 1) {
                if (this.isOperator(s)) {
                    buffer.append(s + " ");
                    continue;
                }
            } else if (isBracket == false & i != 0) {
                sb.append(this.transform(buffer.toString()));
                buffer.setLength(0);
                i--;
                if (bracket >= 1) {
                    isBracket = true;
                }
                while (!operators.empty()) {
                    sb.append(operators.pop());
                }
                if (i == 0) {
                    if (!operators.empty()) {
                        sb.append(operators.pop() + " ");
                    }
                }
                continue;
            }
            if (isBracket == false & buffer.length() == 0) {
                if (this.isOperator(s)) {
                    operators.push(s + " ");
                    continue;
                } else {
                    buffer.append(s + " ");
                }
            } else {
                buffer.append(s + " ");
            }
        }
        if (buffer.length() != 0) {
            sb.append(this.transform(buffer.toString()));
        }
        while (operators.size() != 0) {
            sb.append(operators.pop());
        }
        return sb.toString();
    }

    /**
     * 将运算式字符串转换成逆波兰表达式 (toReversePolishNotation)
     * 不能传入括号 "(",")"
     * 例如: 5 * 2 + 1 返回: 5 2 * 1 +
     * @param expression 运算式
     * @return ReversePolishNotation
     */
    private String transform(String expression) {
        String[] split = expression.split(" ");
        StringBuffer sb = new StringBuffer();
        Stack<String> operators = new Stack<String>();
        for (String s : split) {
            if (s.matches("\d")) {
                sb.append(s + " ");
            } else if (s.equals("+") || s.equals("-")) {
                if (!operators.empty()) {
                    sb.append(operators.pop() + " ");
                }
            }
            if (operators.size() == 2) {
                sb.append(operators.pop() + " ");
            }
            if (this.isOperator(s)){
                operators.push(s);
            }
        }
        while (!operators.empty()) {
            sb.append(operators.pop() + " ");
        }
        return sb.toString();
    }

    /**
     * 判断是否是四则运算符
     * @param operators
     * @return
     */
    private Boolean isOperator(String operators) {
        if (operators.equals("+") || operators.equals("-") || operators.equals("*") || operators.equals("/")) {
            return true;
        } else {
            return false;
        }
    }

}

测试代码

    public static void main(String[] args) {
        // 1 2 + 1 * 1 2 + * 3 1 / -
        String exp = "( ( 1 + 2 ) * 1 ) * ( 1 + 2 ) - ( 3 / 1 )";

        // 1 2 + 1 * 2 1 * +
        //String exp = "( ( ( 1 + 2 ) * 1 ) + ( 2 * 1 ) )";

        // 1 2 + 3 /
        //String exp = "( 1 + 2 ) / 3";

        PolishNotationFactory pnf = new PolishNotationFactory();
        System.out.println("R>>>:" + pnf.toRPN(exp));
    }