Find the number obtained by concatenating binary representations of all numbers up to N
Given an integer N, the task is to find the decimal value of the binary string formed by concatenating the binary representations of all numbers from 1 to N sequentially.
Input: N = 12
Explanation: The concatenation results in “1101110010111011110001001101010111100”. The equivalent decimal value is 118505380540.
Input: N = 3
Explanation: In binary, 1, 2, and 3 correspond to “1”, “10”, and “11”. Their concatenation results in “11011”, which corresponds to the decimal value of 27.
Approach: The idea is to iterate over the range [1, N]. For every ith number, concatenate the binary representation of the number i using the Bitwise XOR property. Follow the steps below to solve the problem:
- Initialize two variables, l, and ans with 0, where l stores the current position of the bit in the final binary string of any ith number and ans will store the final answer.
- Iterate from i = 1 to N + 1.
- If (i & ( i – 1 )) is equal to 0, then simply increment the value of l by 1, where & is the Bitwise AND operator.
- After that, the left shift ans by l and then bitwise OR the result with i.
- After and traversing, print ans as the answer.
Below is the implementation of the above approach:
Time Complexity: O(N)
Auxiliary Space: O(1)