What is the Decode Ways pattern?

The Decode Ways is a common coding interview pattern in Dynamic Programming problems. It provides a structured approach to solving related algorithm challenges efficiently.

When should I use Decode Ways?

Use the Decode Ways pattern when you encounter problems matching its characteristics. Check the "When to Use" and "Clues" sections in the lesson for specific indicators.

Is Decode Ways asked in FAANG interviews?

Yes, Decode Ways is an essential pattern commonly tested in FAANG and top tech company technical interviews.

Dynamic Programming

Medium

Decode Ways

Determine the total number of ways to decode a string of digits into letters (A-Z).

10 min read

Solve on LeetCode

Problem Understanding

We convert digits to letters: '1' -> 'A', '2' -> 'B', ..., '26' -> 'Z'. Given a string s, finding the count of possible decodings.

Example: s = "12"

"1, 2" -> "AB"
"12" -> "L"
Output: 2

Algorithm Strategy

We use DP. Let dp[i] be the number of ways to decode the prefix s[0...i-1].

Transitions:

One Digit: If s[i-1] is '1'-'9', we can add dp[i-1] ways.
Two Digits: If s[i-2...i-1] is "10"-"26", we can add dp[i-2] ways.

Base case: dp[0] = 1 (empty string has 1 way).

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