0483.smallest-good-base

483. Smallest Good Base

题目

For an integer n, we call k>=2 a good base of n, if all digits of n base k are 1. Now given a string representing n, you should return the smallest good base of n in string format.

把 n 转换成 k 进制后，新的数全是 1

Example 1:

Input: "13"
Output: "3"
Explanation: 13 base 3 is 111.

Example 2:

Input: "4681"
Output: "8"
Explanation: 4681 base 8 is 11111.

Example 3:

Input: "1000000000000000000"
Output: "999999999999999999"
Explanation: 1000000000000000000 base 999999999999999999 is 11.

Note:

1. The range of n is [3, 10^18].
2. The string representing n is always valid and will not have leading zeros.

解题思路

见程序注释

func smallestGoodBase(n string) string {
    num, _ := strconv.Atoi(n)

    mMax := int(math.Log2(float64(num)))

    for m := mMax; 1 < m; m-- {
        k := int(math.Pow(float64(num), 1.0/float64(m)))

        tmp := int(math.Pow(float64(k), float64(m)+1)-1) / (k - 1)

        if tmp == num {
            return strconv.Itoa(k)
        }
    }

    return strconv.Itoa(num - 1)
}

以上程序在逻辑和数学上都是完全正确的，但是无法得出正确的解。 例如，当输入"16035713712910627"，其解为"502"，但是当　for 循环中的 k == 502 时，temp == 16035713712910626 与 num 不相等。这是由于精度不够引起的错误。