AI019. 标注序列还原计数

# 序列 DP:每位问「单独成编号(1-9)?与前一位组两位(10-26)?」
# 转移同爬楼梯骨架 dp[i]=dp[i-1]+dp[i-2],条件不满足的支路记 0;全程取模 1e9+7
import sys

MOD = 1_000_000_007


def solve() -> None:
    s = sys.stdin.readline().strip()
    if not s or s[0] == "0":
        print(0)
        return
    prev2, prev1 = 1, 1
    for i in range(1, len(s)):
        cur = 0
        if s[i] != "0":
            cur = prev1
        two = int(s[i - 1:i + 1])
        if 10 <= two <= 26:
            cur = (cur + prev2) % MOD
        prev2, prev1 = prev1, cur
    print(prev1 % MOD)


if __name__ == "__main__":
    solve()