第一题

static String toHex(int n) {    Deque<Integer> deque = new ArrayDeque<>();    char[] hexMap = {'0','1','2','3','4','5','6','7','8','9','A','B','C','D','E','F'};    while (n != 0) {        deque.push(n % 16);        n = n / 16;    }    StringBuilder sb = new StringBuilder();    while (!deque.isEmpty()) {        sb.append(hexMap[deque.pop()]);    }    return sb.toString();}

第二题

public class SuffixA {    public static void main(String[] args) {        String exp = "1 + 2 * (9 - 5)";        SuffixExpressionA se = compile(exp);        int result = se.execute();        System.out.println(exp + " = " + result + " " + (result == 1 + 2 * (9 - 5) ? "✓" : "✗"));    }    static SuffixExpressionA compile(String exp) {        // TODO:        HashMap<Character, Integer> precedence = new HashMap<>();        precedence.put('+', 0);        precedence.put('-', 0);        precedence.put('*', 1);        precedence.put('/', 1);        precedence.put('(', -1);        exp = exp.replaceAll("\\s","");        Deque<Character> operator = new ArrayDeque<>();        String result = "";        for (char c:exp.toCharArray()) {            if (Character.isDigit(c)) {                result += c;            } else if (c == '(') {                operator.push(c);            } else if (c == ')') {                // remove parentheses and output operators inside parentheses                while (operator.peek() != '(') {                    result += operator.pop();                }                operator.pop();            } else if (precedence.containsKey(c)) {                    // output operators with lower/equal precedence                    while (!operator.isEmpty() && precedence.get(c) <= precedence.get(operator.peek())) {                        if (operator.peek() == '(') throw new IllegalArgumentException();                        result += operator.pop();                    }                    // push income operator to stack                    operator.push(c);                } else throw new IllegalArgumentException();        }        while (!operator.isEmpty()) {            result += operator.pop();        }        return new SuffixExpressionA(result);    }}class SuffixExpressionA {    private String exp;    public SuffixExpressionA (String expAftCom) {        this.exp = expAftCom;    }    int execute() {        Deque<Integer> operand = new ArrayDeque<>();        for (char c:exp.toCharArray()) {            if (Character.isDigit(c)) {                operand.push(Character.getNumericValue(c));            } else {                operand.push(cal(c, operand.pop(), operand.pop()));            }        }        if (operand.size() > 1) throw new IllegalArgumentException();        return operand.pop();    }    private int cal(char o, int b, int a) {        switch (o) {            case '+' -> {                return a + b;            }            case '-' -> {                return a - b;            }            case '*' -> {                return a * b;            }            case '/' -> {                return a / b;            }        }        throw new IllegalArgumentException();    }}

第三题

public class SuffixB {    public static void main(String[] args) {        String exp = "x + 2 * (y - 5)";        SuffixExpressionB se = compile(exp);        Map<String, Integer> env = Map.of("x", 1, "y", 9);        int result = se.execute(env);        System.out.println(exp + " = " + result + " " + (result == 1 + 2 * (9 - 5) ? "✓" : "✗"));    }    static SuffixExpressionB compile(String exp) {        HashMap<Character, Integer> precedence = new HashMap<>();        precedence.put('+', 0);        precedence.put('-', 0);        precedence.put('*', 1);        precedence.put('/', 1);        precedence.put('(', -1);        exp = exp.replaceAll("\\s","");        Deque<Character> operator = new ArrayDeque<>();        String result = "";        for (char c:exp.toCharArray()) {            if (Character.isDigit(c) || c == 'x' || c == 'y') {                result += c;            } else if (c == '(') {                operator.push(c);            } else if (c == ')') {                // remove parentheses and output operators inside parentheses                while (operator.peek() != '(') {                    result += operator.pop();                }                operator.pop();            } else if (precedence.containsKey(c)) {                // output operators with lower/equal precedence                while (!operator.isEmpty() && precedence.get(c) <= precedence.get(operator.peek())) {                    if (operator.peek() == '(') throw new IllegalArgumentException();                    result += operator.pop();                }                // push income operator to stack                operator.push(c);            } else throw new IllegalArgumentException();        }        while (!operator.isEmpty()) {            result += operator.pop();        }        return new SuffixExpressionB(result);    }}class SuffixExpressionB {    private String exp;    public SuffixExpressionB (String expAftCom) {        this.exp = expAftCom;    }    int execute(Map<String, Integer> env) {        // replace variables with values        Set<String> var = env.keySet();        for (String s:var) {            exp = exp.replaceAll(s, String.valueOf(env.get(s)));        }        // same calculation as SuffixA        Deque<Integer> operand = new ArrayDeque<>();        for (char c:exp.toCharArray()) {            if (Character.isDigit(c)) {                operand.push(Character.getNumericValue(c));            } else {                operand.push(cal(c, operand.pop(), operand.pop()));            }        }        if (operand.size() > 1) throw new IllegalArgumentException();        return operand.pop();    }    private int cal(char o, int b, int a) {        switch (o) {            case '+' -> {                return a + b;            }            case '-' -> {                return a - b;            }            case '*' -> {                return a * b;            }            case '/' -> {                return a / b;            }        }        throw new IllegalArgumentException();    }}