swap_case_using_n.cpp

//Your job is to change the given string s using a non-negative integer n.

//Each bit in n will specify whether or not to swap the case for each alphabetic character in s: if the bit is 1, swap the case; if its 0, leave it as is. When you finish with the last bit of n, start again with the first bit.

//You should skip the checking of bits when a non-alphabetic character is encountered, but they should be preserved in their original positions.

//Examples
//swap("Hello world!", 11)  -->  "heLLO wORLd!"
//...because 11 is 1011 in binary, so the 1st, 3rd, 4th, 5th, 7th, 8th and 9th alphabetical characters have to be swapped:

//H e l l o   w o r l d !
//1 0 1 1 1 x 0 1 1 1 0 x
//^   ^ ^ ^    ^ ^ ^

#include <string>

std::string swap(std::string s, int n) {
    std::vector<int> bits;
    while (n) {
      bits.insert(bits.begin(), n & 1);
      n >>= 1;
    }
  
    if (!bits.size())
      return s;
  
    std::string result = "";
    int i = 0;
    for (char c : s) {
        if (isalpha(c) && bits[i++ % bits.size()]) {
            c = c < 97 ? tolower(c) : toupper(c);
        }
      
        result += c;
    }
  
    return result;
}