関数を早めに抜ける

main.swift

private func isSimilar(str1: String, str2: String) -> String {
    var ok: String = "Yes"
    // 条件1のチェック
    if (str1.count != str2.count) {
        ok = "No"
        return ok
    }
    
    // 条件2のチェック
    var diffCount = 0
    for index in 0..<str1.count {
        if str1[str1.index(str1.startIndex, offsetBy: index)] != str2[str2.index(str2.startIndex, offsetBy: index)] {
            diffCount += 1
        }
    }
    if diffCount != 1 {
        ok = "No"
    }
    return ok
}


func main() {
    let array = [
        ["aruru", "araru"],
        ["aruru", "arara"],
        ["aruru", "aruru"],
        ["algo", "all"],
        ["str", "std"]
    ]
    
    for it in array {
        print(isSimilar(str1: it[0], str2: it[1]))
    }
    
}
main()