LeetCode problem #91: Decode ways

leetcode-91-decode-ways.rb

# @param {String} s
# @return {Integer}
def num_decodings(s)
    return 0 if s.empty?

    n1 = 1
    n2 = 0
    i = 0

    while i < s.size
        next2 = s[i, 2]
        if next2 == "10" || next2 == "20"
            n1, n2 = 0, n1 + n2
            i += 2
            next
        end

        next1 = s[i].to_i
        return 0 if next1 == 0

        if i > 0
            prev1 = s[i - 1].to_i
            if prev1 == 1 || (prev1 == 2 && (next1 >= 1 && next1 <= 6))
                n1, n2 = n1 + n2, n1
            else
                n1, n2 = n1 + n2, 0
            end
        end

        i += 1
    end

    n1 + n2
end