Problem statement

Given an integer n, return the decimal value of the binary string formed by concatenating the binary representations of 1 to n in order, modulo 10^9 + 7.

Example 1

Input: n = 1
Output: 1
Explanation: "1" in binary corresponds to the decimal value 1.

Example 2

Input: n = 3
Output: 27
Explanation: In binary, 1, 2, and 3 corresponds to "1", "10", and "11".
After concatenating them, we have "11011", which corresponds to the decimal value 27.

Example 3

Input: n = 12
Output: 505379714
Explanation: The concatenation results in "1101110010111011110001001101010111100".
The decimal value of that is 118505380540.
After modulo 10^9 + 7, the result is 505379714.

Constraints

• 1 <= n <= 10^5.

Solution: Recursive

There must be some relationship between the result of n and the result of n - 1.

First, let us list some first values of n.

• For n = 1: the final binary string is "1", its decimal value is 1.

• For n = 2: the final binary string is "110", its decimal value is 6.

• For n = 3: the final binary string is "11011", its decimal value is 27.

Look at n = 3, you can see the relationship between the decimal value of "11011" and the one of "110" (of n = 2) is:

27 = 6 * 2^2 + 3
Dec("11011") = Dec("110") * 2^num_bits("11") + Dec("11")
Result(3) = Result(2) * 2^num_bits(3) + 3.

The same equation for n = 2:

6 = 1 * 2^2 + 2
Dec("110") = Dec("1") * 2^num_bits("10") + Dec("10")
Result(2) = Result(1) * 2^num_bits(2) + 2.

In general, the recursive relationship between n and n - 1 is:

Result(n) = Result(n - 1) * 2^num_bits(n) + n.

Code

#include <cmath>
#include <iostream>
int concatenatedBinary(int n) {
    unsigned long long result = 1;
    for (int i = 2; i <= n; i++) {
        const int num_bits = std::log2(i) + 1;        
        result = ((result << num_bits) + i) % 1000000007;
    }
    return result;
}
int main() {
    std::cout << concatenatedBinary(1) << std::endl;
    std::cout << concatenatedBinary(3) << std::endl;
    std::cout << concatenatedBinary(12) << std::endl;
}

Output:
1
27
505379714

Complexity

• Runtime: O(n).

• Extra space: O(1).

References

• https://leetcode.com/problems/concatenation-of-consecutive-binary-numbers/

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