import (
    "unicode"
)

// Solution struct for encapsulating the algorithm
type Solution struct{}

func (s Solution) findLongestNiceSubstring(str string) string {
    var result string
    for i := 0; i < len(str); i++ {
       var lowerBits, upperBits int
       for j := i; j < len(str); j++ {
          if unicode.IsLower(rune(str[j])) {
             lowerBits |= 1 << (str[j] - 'a')
          } else {
             upperBits |= 1 << (str[j] - 'A')
          }
          if lowerBits == upperBits && j-i+1 > len(result) {
             result = str[i : j+1]
          }
       }
    }
    return result
}