Today I have done leetcode's October LeetCoding Challenge with cpp.

October LeetCoding Challenge 19

Description

Find Kth Bit in Nth Binary String

Given two positive integers n and k, the binary string Sn is formed as follows:

• S1 = "0"
• Si = Si - 1 + "1" + reverse(invert(Si - 1)) for i > 1

Where + denotes the concatenation operation, reverse(x) returns the reversed string x, and invert(x) inverts all the bits in x (0 changes to 1 and 1 changes to 0).

For example, the first four strings in the above sequence are:

• S1 = "0"
• S2 = "011"
• S3 = "0111001"
• S4 = "011100110110001"

Return the kth bit in Sn. It is guaranteed that k is valid for the given n.

Example 1:

Input: n = 3, k = 1
Output: "0"
Explanation: S3 is "0111001".
The 1st bit is "0".

Example 2:

Input: n = 4, k = 11
Output: "1"
Explanation: S4 is "011100110110001".
The 11th bit is "1".

Constraints:

• 1 <= n <= 20
• 1 <= k <= 2n - 1

Solution

class Solution {
public:
  char findKthBit(int n, int k) {
    if(n == 1) return '0';
    int midBit = 1 << (n - 1);
    if(k < midBit) return findKthBit(n - 1, k);
    if(k == midBit) return '1';
    return 1 ^ findKthBit(n - 1, 2 * midBit - k); //if (k > midBit)) 
  }
};