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(","))
})
})