30. Find 1 + 2 + 3 + ... + n (multiplication and division, loop, if, switch cannot be used)

1. Recursive calculation and sum

package com.fy;

public class T4 {
    
    
    public static void main(String[] args) {
    
    
        //创建对象
        T4 s = new T4();
        System.out.println(s.sum(100));
    }
    //sum方法
    public int sum(int n) {
    
    
        //增加判断语句，如果n为1,不添加if语句会进入死循环，进而栈溢出
        if (n == 1) {
    
    
            return 1;
        }
        return n + sum(n - 1);
    }
}