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483. Smallest Good Base

Description

Given an integer n represented as a string, return the smallest good base of n.

We call k >= 2 a good base of n, if all digits of n base k are 1's.

 

Example 1:

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

Example 2:

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

Example 3:

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

 

Constraints:

  • n is an integer in the range [3, 1018].
  • n does not contain any leading zeros.

Solutions

Solution 1

Thinking

The smallest base \(k\ge 2\) in which \(n\) is all ones. \(k=n-1\) always works (\(11_k\)), but \(n\) can be \(10^{18}\), so scanning \(k\) is impossible.

The number of ones \(m+1\) satisfies \(m<60\). Try \(m\) from large to small and binary-search \(k\) so that \(1+k+\cdots+k^m=n\). A larger \(m\) yields a smaller \(k\), so going downward finds the smallest base first.

The geometric sum is monotone in \(k\), so the binary search is valid. If none hits, fall back to \(n-1\).

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class Solution:
    def smallestGoodBase(self, n: str) -> str:
        def cal(k, m):
            p = s = 1
            for i in range(m):
                p *= k
                s += p
            return s

        num = int(n)
        for m in range(63, 1, -1):
            l, r = 2, num - 1
            while l < r:
                mid = (l + r) >> 1
                if cal(mid, m) >= num:
                    r = mid
                else:
                    l = mid + 1
            if cal(l, m) == num:
                return str(l)
        return str(num - 1)

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class Solution {
    public String smallestGoodBase(String n) {
        long num = Long.parseLong(n);
        for (int len = 63; len >= 2; --len) {
            long radix = getRadix(len, num);
            if (radix != -1) {
                return String.valueOf(radix);
            }
        }
        return String.valueOf(num - 1);
    }

    private long getRadix(int len, long num) {
        long l = 2, r = num - 1;
        while (l < r) {
            long mid = l + r >>> 1;
            if (calc(mid, len) >= num)
                r = mid;
            else
                l = mid + 1;
        }
        return calc(r, len) == num ? r : -1;
    }

    private long calc(long radix, int len) {
        long p = 1;
        long sum = 0;
        for (int i = 0; i < len; ++i) {
            if (Long.MAX_VALUE - sum < p) {
                return Long.MAX_VALUE;
            }
            sum += p;
            if (Long.MAX_VALUE / p < radix) {
                p = Long.MAX_VALUE;
            } else {
                p *= radix;
            }
        }
        return sum;
    }
}

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class Solution {
public:
    string smallestGoodBase(string n) {
        long v = stol(n);
        int mx = floor(log(v) / log(2));
        for (int m = mx; m > 1; --m) {
            int k = pow(v, 1.0 / m);
            long mul = 1, s = 1;
            for (int i = 0; i < m; ++i) {
                mul *= k;
                s += mul;
            }
            if (s == v) {
                return to_string(k);
            }
        }
        return to_string(v - 1);
    }
};

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