CodingTour —

ARTS #115

Algorithm

本周选择的算法题是：Decode Ways。

规则

A message containing letters from A-Z can be encoded into numbers using the following mapping:

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

To decode an encoded message, all the digits must be grouped then mapped back into letters using the reverse of the mapping above (there may be multiple ways). For example, "11106" can be mapped into:

• "AAJF" with the grouping (1 1 10 6)
• "KJF" with the grouping (11 10 6)

Note that the grouping (1 11 06) is invalid because "06" cannot be mapped into 'F' since "6" is different from "06".

Given a string s containing only digits, return the 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: "12" could be decoded as "AB" (1 2) or "L" (12).

Example 2:

Input: s = "226"
Output: 3
Explanation: "226" 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.
The only valid mappings with 0 are 'J' -> "10" and 'T' -> "20", neither of which start with 0.
Hence, there are no valid ways to decode this since all digits need to be mapped.

Example 4:

Input: s = "06"
Output: 0
Explanation: "06" cannot be mapped to "F" because of the leading zero ("6" is different from "06").

Constraints:

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

Solution

class Solution:
    def numDecodings(self, s: str) -> int:
        dp = [0] * (len(s) + 1)
        dp[len(s)] = 1
        for i in range(len(s) - 1, -1, -1):
            if s[i] != '0':
                dp[i] = dp[i+1]
                if i < len(s) - 1 and (s[i] == '1' or (s[i] == '2' and s[i+1] < '7')):
                    dp[i] += dp[i + 2]
        return dp[0]

Review

You don’t need Feature Branches anymore…

TBD(Trunk Based Development) 工作流没有 Feature 分支、没有 PR，所有人工作在同一个分支(如 master)，这有一些好处：

• 更低的错误恢复时间 - Mean Time to Recovery 也是一个工作流的关键指标之一

• “强迫”结对编程 - 因为没有 PR 了，为了实现 “4 Eye Principl” 结对成了必须的事，结对编程好处很明显

  • 由于是两位工程师携手解决同一个问题，他们往往有不一样的经验、技术栈和擅长的领域，因此结对是很好的机会去相互学习
  • 不必等到功能开发完成才去 review，事实上在结对的过程中，即时学习(如老带新)、设计决策、团队规范等工作就在不断的反馈中完成了

还有 “confidence and trust”，工程师可以在 TBD 工作流中感觉到被信任。

如果团队有完善的自动化测试流程和相应的监控措施以及完备的线上管理平台(如功能开关、动态化、热修复等)，那 TBD 倒是一个可尝试的方向。

Tip

git-secrets: 一个检测提交内容是否包括安全凭证的工具，它使用了三个 git hooks 实现该功能：

• pre-commit
• commit-msg
• prepare-commit-msg

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