DecodeWays.kt

/**
 * A message containing letters from A-Z is being encoded to numbers using the following mapping:
 *
 * 'A' -> 1
 * 'B' -> 2
 * ...
 * 'Z' -> 26
 * Given an encoded message containing digits, determine the total number of ways to decode it.
 *
 * For example,
 * Given encoded message "12", it could be decoded as "AB" (1 2) or "L" (12).
 *
 * The number of ways decoding "12" is 2.
 *
 * Accepted.
 */
class DecodeWays {

    fun numDecodings(s: String): Int {
        if (s.isEmpty() || s.elementAt(0) == '0') {
            return 0
        }

        val ints = IntArray(s.length + 1)
        ints[0] = 1

        for (i in 1 until ints.size) {
            ints[i] = if (s[i - 1] == '0') 0 else ints[i - 1]

            if (i > 1 && (s[i - 2] == '1' || s[i - 2] == '2' && s[i - 1] <= '6')) {
                ints[i] += ints[i - 2]
            }
        }

        return ints[s.length]
    }

}