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.

class Solution:
    def numDecodings(self, s: str) -> int:
        if not s or s[0] == "0":
            return 0

        dp = [0] * (len(s) + 1)
        dp[0] = dp[1] = 1
        for i in range(2, len(s) + 1):
            if s[i - 1] != '0':
                dp[i] += dp[i - 1]
            if 10 <= int(s[i-2: i]) <= 26:
                dp[i] += dp[i - 2]
        return dp[-1]