Saved from https://leetcode-cn.com/problems/letter-case-permutation/submissions/

letterCasePermutation.go

func isAlpha(c byte) bool {
    return c >= 'a' && c <= 'z' || c >= 'A' && c <= 'Z'
}

func toLowerOrUpper(c byte) byte {
    if c >= 'a' && c <= 'z' {
        return c - 'a' + 'A'
    } else if c >= 'A' && c <= 'Z' {
        return c - 'A' + 'a'
    }
    return c
}

func letterCasePermutation(S string) []string {
    sumC := uint(0)
    for i := 0; i < len(S); i++ {
        if isAlpha(S[i]) {
            sumC++
        }
    }
    res := make([]string, 0)
    s := make([]byte, len(S))
    for i := 0; i < (1 << sumC); i++ {
        cur := uint(0)
        for j := 0; j < len(S); j++ {
            if isAlpha(S[j]) {
                if i&(1<<cur) != 0 {
                    s[j] = toLowerOrUpper(S[j])
                } else {
                    s[j] = S[j]
                }
                cur++
            } else {
                s[j] = S[j]
            }
        }
        res = append(res, string(s))
    }
    return res
}