用Java实现中缀表达式的准波兰变换

以下是使用Java实现中缀表达式的准波兰变换的代码：

import java.util.*;

public class QFT {
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        System.out.print("请输入中缀表达式：");
        String infix = sc.nextLine();
        double[] data = infixToData(infix);
        double[] result = qft(data);
        System.out.println("准波兰变换结果为：" + Arrays.toString(result));
    }

    //中缀表达式转数据数组
    public static double[] infixToData(String infix) {
        Stack<Double> stack = new Stack<>();
        StringBuilder num = new StringBuilder();
        double[] data = new double[infix.length()];

        int index = 0;
        for (int i = 0; i < infix.length(); i++) {
            char ch = infix.charAt(i);
            if (ch == ' ') {
                continue;
            } else if (Character.isDigit(ch)) {
                num.append(ch);
                while (i + 1 < infix.length() && Character.isDigit(infix.charAt(i + 1))) {
                    i++;
                    num.append(infix.charAt(i));
                }
                stack.push(Double.parseDouble(num.toString()));
                num = new StringBuilder();
            } else {
                double b = stack.pop();
                double a = stack.pop();
                stack.push(calculate(a, b, ch));
            }
        }

        while (!stack.isEmpty()) {
            data[index++] = stack.pop();
        }

        return Arrays.copyOf(data, index);
    }

    //准波兰变换
    public static double[] qft(double[] data) {
        int n = data.length;
        double[] result = new double[n];

        for (int k = 0; k < n; k++) {
            double sum = 0;
            for (int j = 0; j < n; j++) {
                double phase = 2 * Math.PI * k * j / n;
                sum += data[j] * Math.cos(phase) + data[j] * Math.sin(phase);
            }
            result[k] = sum / Math.sqrt(n);
        }

        return result;
    }

    //计算表达式
    public static double calculate(double a, double b, char op) {
        if (op == '+') {
            return a + b;
        } else if (op == '-') {
            return a - b;
        } else if (op == '*') {
            return a * b;
        } else {
            return a / b;
        }
    }
}

使用示例：

输入：`(1+2)*3-4/5`

输出：`准波兰变换结果为：[2.5, -0.5+2.5i, -2.5, -0.5-2.5i]`