Java 递归方法 - findMax 和 countPositive

Question

我遇到了一些递归方法的问题。我正在编写一个程序，它从标准输入读取整数序列，直到读取到 0，并将它们存储在数组中（包括 0）。 findMax 一直返回 0，而 countPositive 和 SumDivisibleBy3 似乎大部分时间都偏离了几个数字。我还需要帮助实现 public static int exists(int[] array, int lastIndex, int searchElement) 来搜索数组。

import java.io.*;

public class Assignment9 {

    public static void main(String[] args) throws IOException {
        int i = 0;
        int [] nums;
        nums = new int [100];
        InputStreamReader inRead = new InputStreamReader(System.in);   
        BufferedReader buffRead = new BufferedReader(inRead);
        String line = buffRead.readLine();
        try {    
            while (line.equals("0") == false && i<100) {        
                i++;        
                line = buffRead.readLine();     
                nums[i]=(int) Double.parseDouble(line);     
            }      
        } catch(IOException e) {        
            System.out.println("Array index out of bound");   
        }

        int findM = findMax(nums, 0, nums.length-1);
        int countP = countPositive(nums, 0, nums.length-1);
        int divis3 = computeSumDivisibleBy3(nums, 0, nums.length-1);

        System.out.println("The maximum number is: " + findM);
        System.out.println("The count of positive integers is " + countP);
        System.out.println("The sum of the numbers divisible by 3 is " + divis3);       
    }

    public static int findMax(int[] numbers, int startIndex, int endIndex) 
    {           
        int max = 0;
        if (startIndex == endIndex) {
            return numbers[startIndex];
        } else if (numbers[startIndex] < numbers[endIndex]) {
            return findMax(numbers, startIndex + 1, endIndex);
        } else {
            return numbers[startIndex];
        }       
    }

    public static int countPositive(int[] numbers, int startIndex, int endIndex) 
    {           
        if (startIndex == endIndex) {   
             if (numbers[startIndex] > 0)        
             {   
                  return 1;
             }   
             else
                  return 0;      
         } else {
           if (numbers[startIndex] > 0)        
           {       
            return 1 + countPositive(numbers, startIndex +1, endIndex); 
           }
           else        
               return countPositive(numbers, startIndex +1, endIndex);
        }
    }

    public static int computeSumDivisibleBy3(int[] numbers, int startIndex, int endIndex) 
    {           
        if (startIndex == numbers.length - 1) {
            return numbers[startIndex] % 3 == 0 ? numbers[startIndex] : 0;
        } else {
            return (numbers[startIndex] % 3 == 0 ? numbers[startIndex] : 0) + computeSumDivisibleBy3(numbers, ++startIndex, endIndex);
        }
    }

//  public static int exists(int[] array, int lastIndex, int searchElement) {

    //}
}

Answer 1

好吧，在你的 findMax 中，你的逻辑是错误的。 您想将第一个整数与最后 n-1 个元素的最大值进行比较：

public static int findMax(int[] numbers, int startIndex, int endIndex) 
{
    if (startIndex == endIndex) {
        return numbers[startIndex];
    } else {           
        int max = findMax(numbers, startIndex + 1, endIndex);
        if (numbers[startIndex] < max) {
            return max;
        } else {
            return numbers[startIndex];
        }
    }       
}

您的countPositive 在我看来是正确的。也许你用错误的输入参数调用它。

你的最后一个方法也是错误的。检查停止条件时，您没有使用endIndex。

应该是：

public static int computeSumDivisibleBy3(int[] numbers, int startIndex, int endIndex) 
{
    if (startIndex == endIndex) {
        return numbers[startIndex] % 3 == 0 ? numbers[startIndex] : 0;
    } else {
        return (numbers[startIndex] % 3 == 0 ? numbers[startIndex] : 0) + computeSumDivisibleBy3(numbers, startIndex + 1, endIndex);
    }
}

我刚刚提出了exists 方法。首先，我将返回类型更改为布尔值。然后我会做一个与你在其他方法中所做的类似的递归 - 对单个元素数组的特殊处理，并将第一个元素的测试与对数组的最后 n - 1 个元素的递归调用相结合。

public static boolean exists(int[] array, int lastIndex, int searchElement) 
{
    if (startIndex == endIndex) {
        return numbers[startIndex] == searchElement;
    } else {
        return (numbers[startIndex] == searchElement) || exists(array, startIndex + 1, endIndex);
    }
}

Answer 2

public static int computeSumDivisibleBy3(int[] numbers, int startIndex, int endIndex)
{
    if (startIndex==endIndex) {
        if(numbers[startIndex]%3==0) {
            return numbers[startIndex];
        } else {
            return 0;
        }
    } else  {
        if(numbers[startIndex]%3==0) {
            return computeSumDivisibleBy3(numbers,startIndex+1, endIndex)+numbers[startIndex];
        } else{
            return  computeSumDivisibleBy3(numbers,startIndex+1, endIndex);
        }
    }
}