# ---
# title: 91. Decode Ways
# id: problem91
# author: Indig
# date: 2021-06-02
# difficulty: Medium
# categories: String, Dynamic Programming
# link: <https://leetcode.com/problems/decode-ways/description/>
# hidden: true
# ---
# 
# 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
    dp[end]
end
## @lc code=end
