https://atcoder.jp/contests/arc113/tasks/arc113_b が「7進法」だった場合の解

main.swift

func readInt3(line: Int = #line, file: String = #file) -> (Int, Int, Int) {
    guard let string = readLine() else {
        preconditionFailure("No input (at line \(line) in \(file))")
    }
    let values: [Int] = string.split(separator: " ").map { part in
        guard let value = Int(part) else {
            preconditionFailure("Illegal input value: \(string) (at line \(line) in \(file))")
        }
        return value
    }
    precondition(values.count == 3, "Illegal number of input values: count = \(values.count), values = \(values) (at line \(line) in \(file))")
    return (values[0], values[1], values[2])
}

func pow<Integer>(_ a: Integer, _ b: Integer, modulus: Integer?) -> Integer where Integer: BinaryInteger {
    var result: Integer = 1
    var a = a
    var b = b
    if let modulus = modulus {
        while true {
            if b & 0x1 != 0 {
                result = (result * a) % modulus
            }
            b >>= 1
            guard b > 0 else { break }
            a = (a * a) % modulus
        }
    } else {
        while true {
            if b & 0x1 != 0 {
                result *= a
            }
            b >>= 1
            guard b > 0 else { break }
            a *= a
        }
    }
    return result
}

let (a, b, c) = readInt3()
var k = pow(b, c, modulus: 6)
if k == 0 { k = 6 }
print(pow(a, k, modulus: 7))

//for a in 1 ..< 10 {
//    for b in 1 ..< 3 {
//        for c in 1 ..< 5 {
//            let ans1 = pow(a, pow(b, c, modulus: nil), modulus: nil)
//            var k = pow(b, c, modulus: 6)
//            if k == 0 { k = 6 }
//            let ans2 = pow(a, k, modulus: 7)
//            if ans1 % 7 != ans2 {
//                print(a, b, c)
//                print(ans1, ans2)
//            }
//        }
//    }
//}