求反阶乘的两种算法

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学习笔记

Inverse Factorial反阶乘

例

3! = 1 * 2 * 3 = 6
4! = 3! * 4 = 24

f(0)=-1
f(1)=1
f(2)=2
f(6)=3
f(24)=4
f(7)=-1

算法1

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public int inverseFactorial(int number) {
    if (number == 1) {
        return 1;
    }
    if (number < 1) {
        return -1;
    }
    int i = 2;
    int s = 1;
    while (s < number) {
        s = s * i;
        i++;
    }
    if (s == number) {
        return i - 1;
    } else {
        return -1;
    }
}

算法2

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public int inverseFactorial(int number) {
    if (number == 1) {
        return 1;
    }
    if (number < 1) {
        return -1;
    }
    int i = 2;
    while (number > 1) {
        if (number % i != 0) {
            return -1;
        }
        number /= i;
        i++;
    }
    return i - 1;
}