题解 | 牛牛学数列

const rl = require("readline").createInterface({ input: process.stdin });

function dcsl_sum(a1, d, n) {
    return n * a1 + (n * (n - 1) * d) / 2;
}

rl.on("line", (input) => {
    //分治思想：n为偶数
    // if ((input * 1) % 2 === 0) {
    //     n = input / 2;
    //     z_sum = dcsl_sum(1, 2, n);
    //     f_sum = dcsl_sum(-2, -2, n);
    //     result = z_sum + f_sum;
    // }
    //n为奇数
    // else {
    //     n = Math.floor(input / 2);
    //     z_sum = dcsl_sum(1, 2, n + 1);
    //     f_sum = dcsl_sum(-2, -2, n);
    //     result = z_sum + f_sum;
    // }

    //优化：使用Math里的floor和ceil动态取值
    n = input / 2;
    z_sum = dcsl_sum(1, 2, Math.ceil(n));
    f_sum = dcsl_sum(-2, -2, Math.floor(n));
    result = z_sum + f_sum;
    console.log(result);
    rl.close();
});