String Transforms Into Another String

String Transforms Into Another String

What is this problem about?

The "String Transforms Into Another String" interview question asks whether it is possible to transform string s1 into string s2 by performing a series of character substitutions. In one operation, you can choose any character in s1 and replace all occurrences of it with another character. The transformation is successful if, after any number of such operations, s1 becomes identical to s2. This problem requires a deep dive into character mapping and the constraints of such transformations, particularly focusing on how many unique characters are available in the character set (usually lowercase English letters).

Why is this asked in interviews?

This question is frequently asked by top-tier companies like TikTok and Google because it tests a candidate's ability to model a problem using graph theory and hash tables. It's not a simple string problem; it's about understanding dependencies. If 'a' maps to 'b', and 'b' maps to 'c', we need to ensure that these mappings don't create impossible situations, like 'a' needing to map to two different characters at once. It also tests the candidate's awareness of system-level constraints, such as the total size of the alphabet, which is a key factor in the solution.

Algorithmic pattern used

The primary pattern for this coding problem involves Hash Tables and Graph Theory. We use a Hash Table to store the mapping from each character in s1 to its corresponding character in s2. If any character in s1 is mapped to more than one unique character in s2, the transformation is impossible. Furthermore, if all 26 characters of the alphabet are used in s2 and s1 is not already equal to s2, a transformation might be impossible because there's no "temporary" character available to break cycles in the mapping graph (e.g., 'a' -> 'b' and 'b' -> 'a').

Example explanation (use your own example)

Suppose we have s1 = "aabcc" and s2 = "ccdee".

1. 'a' in s1 maps to 'c' in s2.
2. 'b' in s1 maps to 'd' in s2.
3. 'c' in s1 maps to 'e' in s2. Every character in s1 maps to exactly one character in s2. Since we have at least one character in the alphabet that is not used in s2 (like 'f' or 'z'), we can use it as a temporary placeholder to avoid overwriting characters during the transformation. Therefore, the result is true. If s2 used all 26 letters and s1 wasn't already s2, we couldn't perform the swap without permanently losing a character's identity.

Common mistakes candidates make

A common mistake is failing to recognize the "26 characters" edge case. Many candidates correctly implement the hash table mapping check but forget that if the target string contains every possible character, you cannot perform any transformations that involve cycles. Another mistake is overcomplicating the problem with complex graph traversal algorithms when a simple mapping and a set to count unique characters in s2 are often enough to solve it.

Interview preparation tip

To prepare for the String Transforms Into Another String interview question, practice thinking about "mapping" problems. Ask yourself: "Can one input have two outputs?" and "Are there any global constraints (like alphabet size) that limit my options?" Understanding the pigeonhole principle is also helpful for explaining the final condition to your interviewer.