面试题 64. 求 1+2+…+n

输入: n = 3
输出: 6

输入: n = 9
输出: 45

class Solution:
    def sumNums(self, n: int) -> int:
        return n and (n + self.sumNums(n - 1))

class Solution {
    public int sumNums(int n) {
        int s = n;
        boolean t = n > 0 && (s += sumNums(n - 1)) > 0;
        return s;
    }
}

/**
 * @param {number} n
 * @return {number}
 */
var sumNums = function (n) {
  return (n ** 2 + n) >> 1;
};

func sumNums(n int) int {
	s := 0
	var sum func(int) bool
	sum = func(n int) bool {
		s += n
		return n > 0 && sum(n-1)
	}
	sum(n)
	return s
}