递归的理解

package test;

//递归
public class test {
    public static void main(String[] args) {
        int sum = sum(3);
        System.out.println(sum);
        f(10);
    }

    public static int sum(int n) {
        if (n <= 1) {
            return n;
        }
        //逐渐返回(当n==1 返回1给sum(2) sum(2)=2+1 sum(2)返回3给sum(3) sum(3)=3+3)
        return n + sum(n - 1);
    }

    public static void f(int n) {
        if (n > 0) {
            f(n - 10);
            f(n - 1);
        }
        //n<0输出n并回溯ex:(栈f(10),f(0))
        //执行到f(0)时(n>0 is false 则输出0,回溯到f(10)继续执行此时栈(f(10),f(9),f(-1)))
        //执行到f(-1)时(n>0 is false 则输出-1,回溯到f(9)继续执行此时栈(f(10),f(9),f(8),f(-2))......
        //执行到最后依次退栈即可
        System.out.print(n + " ");//0 -1 -2 -3 -4 -5 -6 -7 -8 -9 0 1 2 3 4 5 6 7 8 9 10 
    }
}