Case permutations of a given word

casePerm.go

package main

import (
	"fmt"
)

func main() {
	key := "hello"
	words := letterCasePermutation(key)
	fmt.Println(words)
}

func letterCasePermutation(S string) []string {
	ans := []string{}
	// Use DFS to generate all letter permutations of string: S.
	// Recursively call to transform one character at a time.
	dfs([]byte(S), 0, &ans)
	return ans
}

// s is the transformed string.
// i is the index of character to be transformed.
func dfs(s []byte, i int, ans *[]string) {
	if i == len(s) {
		// All characters have been transformed, add it to answers.
		*ans = append(*ans, string(s))
		return
	}

	// Original string.
	dfs(s, i+1, ans)

	// Transform string, if character at index: i is an alphabet.
	if isAlphabet(s[i]) {
		s[i] ^= (1 << 5)
		dfs(s, i+1, ans)
	}
}

func isAlphabet(c byte) bool {
	if (c >= 65 && c <= 90) || (c >= 97 && c <= 122) {
		return true
	}
	return false
}