Ask for 1+2+3+...+n, the request cannot use multiplication method, for, while, if, else, switch, case and other keywords and conditional judgment statement (A? B:C).

Method 1: Recursive, using logic and short-circuit characteristics

 Public class Solution {    int sum_solution (int  n) {        int ans = n;         && (ans + = sum_solution (n-1))        ; return ans;    }}

Act 2. Calling Java library functions

 Public class Solution {    publicint sum_solution (int  n) {        int Sum = ( C10>int) (Math.pow (n,2) + n);        return sum>>1;}    }

Ask for 1+2+3+...+n, the request cannot use multiplication method, for, while, if, else, switch, case and other keywords and conditional judgment statement (A? B:C).