91. Decode Ways

A message containing letters from A-Z is being encoded to numbers using the following mapping:

'A' -> 1
'B' -> 2
...
'Z' -> 26

Given a non-empty string containing only digits, determine the total number of ways to decode it.

The answer is guaranteed to fit in a 32-bit integer.

Example 1:

Input: s = "12"
Output: 2
Explanation: It could be decoded as "AB" (1 2) or "L" (12).

Example 2:

Input: s = "226"
Output: 3
Explanation: It could be decoded as "BZ" (2 26), "VF" (22 6), or "BBF" (2 2 6).

Example 3:

Input: s = "0"
Output: 0
Explanation: There is no character that is mapped to a number starting with '0'. We cannot ignore a zero when we face it while decoding. So, each '0' should be part of "10" --> 'J' or "20" --> 'T'.

Example 4:

Input: s = "1"
Output: 1

Constraints:

• 1 <= s.length <= 100
• s contains only digits and may contain leading zero(s).

# @lc code=start
using LeetCode

function num_decodings(s)
    len = length(s)
    dp = fill(0, len + 1)
    dp[1] = 1
    for i in 1:len
        s[i] != '0' && (dp[i + 1] += dp[i])
        (i > 1 && s[i - 1] != '0' && parse(Int, s[(i - 1):i]) <= 26) &&
            (dp[i + 1] += dp[i - 1])
    end
    return dp[end]
end
# @lc code=end

num_decodings (generic function with 1 method)

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