The challenge

Write a function named first_non_repeating_letter that takes a string input, and returns the first character that is not repeated anywhere in the string.

For example, if given the input 'stress', the function should return 't', since the letter t only occurs once in the string, and occurs first in the string.

As an added challenge, upper- and lowercase letters are considered the same character, but the function should return the correct case for the initial letter. For example, the input 'sTreSS' should return 'T'.

If a string contains all repeating characters, it should return an empty string ("") or None — see sample tests.

The solution in Golang

Option 1:

package solution
import (
  "strings"
)
func FirstNonRepeating(str string) string {
    for _, c := range str {
        if strings.Count(strings.ToLower(str), strings.ToLower(string(c))) < 2 {
            return string(c)
        }
    }
    return ""
}

Option 2:

package solution
func FirstNonRepeating(str string) string {
  seen := make(map[rune]int)
  for _, r := range str {
    seen[r|32]++
  }
  for _, r := range str {
    if seen[r|32] == 1 {
      return string(r)
    }
  }
  return ""
}

Option 3:

package solution
import "strings"
func FirstNonRepeating(str string) string {
  s := strings.ToLower(str)
  for i := 0; i < len(s); i++ {
    st := string(s[i])
    if strings.Index(s, st) == strings.LastIndex(s, st) {
      return string(str[i])
    }
  }
  return ""
}

Test cases to validate our solution

package solution_test
import (
  . "github.com/onsi/ginkgo"
  . "github.com/onsi/gomega"
)
var _ = Describe("Basic Tests", func() {
   It("should handle simple tests", func() {
     Expect(FirstNonRepeating("a")).To(Equal("a"))
     Expect(FirstNonRepeating("stress")).To(Equal("t"))
     Expect(FirstNonRepeating("moonmen")).To(Equal("e"))
   })
    It("should handle empty strings", func() {
     Expect(FirstNonRepeating("")).To(Equal(""))
    })
    It("should handle all repeating strings", func() {
     Expect(FirstNonRepeating("abba")).To(Equal(""))
     Expect(FirstNonRepeating("aa")).To(Equal(""))
    })
    It("should handle odd characters", func() {
     Expect(FirstNonRepeating("~><#~><")).To(Equal("#"))
     Expect(FirstNonRepeating("hello world, eh?")).To(Equal("w"))
    })
    It("should handle letter cases", func() {
     Expect(FirstNonRepeating("sTreSS")).To(Equal("T"))
     Expect(FirstNonRepeating("Go hang a salami, I'm a lasagna hog!")).To(Equal(","))
    })
})