使用Java將Infix表達式轉換為前綴和后綴表達式

Question

我想制作一個將表達式從中綴模式轉換為后綴和前綴模式的應用程序。

例如：

中綴：a + b * c

后綴：abc * +

前綴：+ a * bc

我需要兩個類，一個用於后綴轉換的類，另一個用於前綴轉換的類。

另外，我已經為堆棧編寫了一個這樣的類：

public class Stack {

int top = 0;
int stackSize = 0;
String[] stack;

public Stack(int stackSize){
    this.stackSize = stackSize;
    stack = new String[stackSize];
}

public int getTop(){
    return top;
}

public void push(String arg)throws Exception{
    if(top == stackSize){
        throw new Exception("Stack is full!");
    }
    else{
        this.stack[top++] = arg;
    }
}

public String pop()throws Exception{
    if(top == 0){
        throw new Exception("Stack is empty");
    }
    else{
        return this.stack[--top];
    }
}

public int stackCount(){
    return top++;
}
}

我嘗試將此代碼作為postfix類：

public class Postfix {
Stack stack = new Stack(1000);
char []operators = {'(','*','%','/','+','-',')'}; 
char[] infixExpression;
public Postfix(String infixExpression){
    this.infixExpression = infixExpression.toCharArray();
}

int priority(char operator){
    int result = 0;
    switch(operator){
    case '(':
        result = 1;
        break;
    case '*':
        result = 2;
        break;
    case '%':
        result = 3;
        break;
    case '/':
        result = 4;
        break;
    case '+':
        result = 5;
        break;
    case '-':
        result = 5;
        break;
    case ')':
        result = 7;
    }
    return result;
}

public String convertToPostfix(){
    int priority;
    String lastData;
    String exp = "";
    for(int i = 0;i<this.infixExpression.length;i++){
        priority = priority(this.infixExpression[i]);
        if(priority == 0){
            exp += this.infixExpression[i];
        }
        else if(priority == 1){
            try {
                stack.push(String.valueOf(this.infixExpression[i]));
            } catch (Exception e) {
                e.printStackTrace();
            }
        }
        else if(priority == 7){
            while(stack.top != 0){
                try {
                    lastData = stack.pop();
                    if(!lastData.equals("(")){
                        exp += lastData;
                    }
                    else{
                        break;
                    }
                } catch (Exception e) {
                    e.printStackTrace();
                }
            }
        }
        else{
            try { 
                if(stack.top != 0){
                    while(true){
                        lastData = stack.pop();
                        stack.push(lastData);
                        if(stack.top == 0 || lastData.equals("(") || priority(lastData.toCharArray()[0]) > priority){
                            stack.push(String.valueOf(this.infixExpression[i]));
                            break;
                        }
                        exp += stack.pop();
                    }
                }else{
                    stack.push(String.valueOf(this.infixExpression[i]));
                }

            } catch (Exception e) {
                e.printStackTrace();
            }
        }
    }
    if(stack.top != 0){
        try {
            while(stack.top != 0){
                exp += stack.pop();
            }
        } catch (Exception e) {
            e.printStackTrace();
        }
    }
    return exp;
}
}

但這不適用於括號。

Answer 1

所有你需要的是：

1. 將輸入表達式拆分為項目，拆分項目為乘數。
2. 按需要的順序添加到緩沖區部分和定界符

一些不好的代碼來做到這一點：

public class Converter {

    private String infix;

    private void addMultPre(StringBuilder builder, String item) {
        String[] multipliers = item.split("[*/]");
        for (int j = 0; j < multipliers.length - 1; j++) {
            builder.append(infix.charAt(builder.length() + multipliers[j].length()));
            builder.append(multipliers[j]);
        }
        builder.append(multipliers[multipliers.length - 1]);
    }

    private void addMultPost(StringBuilder builder, String item) {
        String[] multipliers = item.split("[*/]");
        builder.append(multipliers[0]);
        for (int j = 1; j < multipliers.length; j++) {
            builder.append(multipliers[j]);
            builder.append(infix.charAt(builder.length()));
        }
    }

    public String prefix() {
        String[] items = infix.split("[+-]");
        StringBuilder builder = new StringBuilder();
        for (int i = 0; i < items.length - 1; i++) {
            builder.append(infix.charAt(builder.length() + items[i].length()));
            addMultPre(builder, items[i]);
        }
        addMultPre(builder, items[items.length - 1]);
        return builder.toString();
    }

    public String postfix() {
        String[] items = infix.split("[+-]");
        StringBuilder builder = new StringBuilder();
        char sign;
        addMultPost(builder, items[0]);
        for (int i = 1; i < items.length; i++) {
            sign = infix.charAt(builder.length());
            addMultPost(builder, items[i]);
            builder.append(sign);
        }
        return builder.toString();
    }

    public Converter(String infix) {
        this.infix = infix;
    }

    public static void main(String[] args) {
        Converter c = new Converter("a+b*c");
        System.out.println(c.postfix());
        System.out.println(c.prefix());
    }
}

輸出：

abc*+
+a*bc