Java implementation of recursion and classic cases

1. Introduction to recursion

Recursion is a widely applied algorithm. In practical development, it can solve specific problems with specific rules. When problems need to be solved, it can be split into several small problems. The same size problem solving method or regular rule is called the method itself in the method definition. All recurring problems can be recursively solved. To solve;

Precautions for using recursion:

• There should be exits, or dead recursion
• The number of times cannot be too many, or the memory will overflow
• Constructor cannot be used recursively
• There are certain rules
• The idea of solving problems recursively is to find out the rules of mouth

2. Factorial case

2.1 circulation mode

public static void main(String[] args) {
    int jc = 1;
    for(int i = 1; i <= 5; i++){
        jc*=i;
    }
    System.out.println("5 The factorials of are:" + jc);//The factorial of 5 is: 120
}

2.2 recursive mode

public static void main(String[] args) {
    System.out.println("5 The factorials of are:" +jiecheng(5));//The factorial of 5 is: 120
}

private static int jiecheng(int i) {
    if(i == 1){
        return 1;
    }else{
        return i*jiecheng(i-1);
    }
}

3. The case of immortal rabbit

3.1 array mode

public static void main(String[] args) {
    //Define an array of 20 months
    int[] arr = new int[20];
    //1 for the first two months
    arr[0] = 1;
    arr[1] = 1;

    //The logarithm of the next month from the third month is the sum of the logarithm of this month
    for(int i = 2; i < arr.length; i++){
        arr[i] = arr[i - 1] + arr[i-2];
    }
    System.out.println("The number of rabbits in the 20th month is:" + arr[19]);//The number of rabbits in the 20th month is 6765
}

3.2 change mode of variables

public static void main(String[] args) {
    //First two months known
    int a = 1;
    int b = 1;

    //For loop traversal
    for(int i = 0; i <18; i++){
        int temp = a;
        a = b;//a value of next month
        b = temp +a;//b value of next month
    }
    System.out.println("The number of rabbits in the 20th month is:" + b);//The number of rabbits in the 20th month is 6765
}

3.3 recursive mode

public static void main(String[] args) {
    System.out.println("The number of rabbits in the 20th month is:" + get(20));//The number of rabbits in the 20th month is 6765
}

private static int get(int i) {
	if(i == 1 || i == 2){//Export conditions
		return 1;
	}else{
		return get(i - 1) + get(i - 2);//law
	}
}

4. Use recursion to delete a file case with content

public static void main(String[] args) {
    //Create File object
    File fileSrc = new File("d:\\kaka") ;
    //Define how to delete a folder
    deleteSrcFolder(fileSrc) ;
}

private static void deleteSrcFolder(File srcFolder) {
    //Get all files under the srcFolder folder and the File array of the folder
    File[] fileArray = srcFolder.listFiles() ;
    //Judge if it is not null, and then traverse
    if(fileArray !=null) {
        for(File file : fileArray) {
            //Determine whether the File object is a folder
            if(file.isDirectory()) {
                //Call recursive delete method
                deleteSrcFolder(file) ;
            }else {
                //Not a folder
                System.out.println(file.getName()+"---"+file.delete());
            }
        }
        System.out.println(srcFolder.getName()+"---"+srcFolder.delete()); //delete() can only delete empty directories
    }
}