题解 | #牛牛学数列3#

知识点：

循环控制：循环控制

#include <iostream>
#include <cmath>
using namespace std;

int main() {
    int n;
    double ret = 0.0, tmp = 0.0;

    scanf("%d", &n);

    for (int i = 1; i <= n; i++) {
        tmp += pow(-1, i - 1) * (2 * i - 1);
        ret += 1.0 / tmp;
    }

    printf("%.3f", ret);
}

/*
int main() {
    int n;
    int down = 0;
    int flag = 1;
    double sum = 0.0;

    scanf("%d", &n);

    for (int i = 1; i <= 2 * n - 1; i = i + 2) {
        down = down + flag * i;
        flag = -1 * flag;
        sum = sum + 1.0 / down;
    }

    printf("%.3f", sum);

    return 0;
}
*/