跟踪递归

Question

我在网上看到了这段代码，我想跟踪递归。我设法做到了：

A 
AB
ABC

我认为我做错了。

这是代码：

public class Test
{
    public static void main(String[] args)  
    {
        printAll("ABC", 0, new StringBuffer());
        //      Outputs
        //      ---------
        //      A
        //      AB
        //      ABC
        //      AC
        //      B
        //      BC    
        //      C
    }

    private static void printAll(String str,int start, StringBuffer sb)
    {
        for(int i=start; i< str.length(); i++)
        {
            sb.append(str.charAt(i));
            System.out.println(sb);
            printAll(str, i+1, sb);
            sb.setLength(sb.length()-1);
        }
    }
}

Answer 1

这是一个简单的“展开”：

输入序列是ABC，所以这样：

for(int i=start; i< str.length(); i++)
{
    sb.append(str.charAt(i));
    System.out.println(sb);
    printAll(str, i+1, sb);
    sb.setLength(sb.length()-1);
}

展开至此，start 设置为 0：

sb.append("ABC".charAt(0));
System.out.println(sb);
printAll("ABC", 1, sb); //Recursive call
sb.setLength(sb.length()-1);
sb.append("ABC".charAt(1));
System.out.println(sb);
printAll("ABC", 2, sb); //Recursive call
sb.setLength(sb.length()-1);
sb.append("ABC".charAt(2));
System.out.println(sb);
printAll("ABC", 3, sb); //Recursive call
sb.setLength(sb.length()-1);

鉴于长度为3，我们到此结束。但现在我们还需要展开递归调用：

sb.append("ABC".charAt(0)); // sb="A"
System.out.println(sb);
//Entering recursive call, start=1
    sb.append("ABC".charAt(1));  // sb="AB"
    System.out.println(sb);

    //Entering recursive call, start=2
        sb.append("ABC".charAt(2));  // sb="ABC"
        System.out.println(sb);
        //Enter recursive call, start=3
            // 3 is not less than "ABC".length(), so we leave immediately
        //Exit recursive call
        sb.setLength(sb.length()-1);  // sb="AB"
    //Exit recursive call

    sb.setLength(sb.length()-1);  // sb="A"
    sb.append("ABC".charAt(2));  // sb="AC"
    System.out.println(sb);
    //Enter recursive call, start=3
        // 3 is not less than "ABC".length(), so we leave immediately
    //Exit recursive call
    sb.setLength(sb.length()-1);  // sb="A"
//Exit recursive call

sb.setLength(sb.length()-1);  // sb=""
sb.append("ABC".charAt(1));  // sb="B"
System.out.println(sb);

//Entering recursive call, start=2
    sb.append("ABC".charAt(2));  // sb="BC"
    System.out.println(sb);
    //Enter recursive call, start=3
        // 3 is not less than "ABC".length(), so we leave immediately
    //Exit recursive call
    sb.setLength(sb.length()-1);  // sb="B"
//Exit recursive call

sb.setLength(sb.length()-1);  // sb=""
sb.append("ABC".charAt(2));  // sb="C"
System.out.println(sb);

//Enter recursive call, start=3
    // 3 is not less than "ABC".length(), so we leave immediately
//Exit recursive call

sb.setLength(sb.length()-1);  // sb=""

有了这个完全“展开”，我们可以将它粘贴到 main 函数中并执行它，看到预期的结果。您还可以像我在 cmets 中一样，了解代码在每个步骤中的作用。