求1+2+3+...+n

递推公式：f(n) = f(n-1) + n
递归终止条件：f(1) = 1

public class Solution {
    public int Sum_Solution(int n) {
        if (n == 1) {
            return 1;
        } else {
            return n + Sum_Solution(n-1);
        }
    }
}