def numDecodings(A):
    if len(A) == 0:
        return 0
    if int(A[0]) == 0:
        return 0
    n = len(A)
    result = [0 for _ in range(0, n + 1)]
    result[0] = result[1] = 1
    for i in range(1, n):
        v1 = int(A[i:i + 1])
        v2 = int(A[i - 1:i + 1])
        if 0 < v1 <= 9:
            result[i + 1] = result[i]
        if 10 <= v2 <= 26:
                result[i + 1] = result[i + 1] + result[i - 1]
        if result[i + 1] == 0:
            return 0

    answer = result[n]
    return answer

print(numDecodings("1204"))