PPractick

The problem

A cipher checker receives a plaintext s and a ciphertext t and must decide whether t could come from s by a consistent one-to-one letter substitution: every occurrence of a letter maps to the same replacement, and no two different letters share a replacement. For example "egg" → "add" works (e→a, g→d), but "foo" → "bar" doesn't (o can't map to both a and r). The mapping has to be checked in both directions; forgetting the reverse direction is the classic bug.

Example: ("paper", "title") → true; ("badc", "baba") → false.

Function signature

bool isIsomorphic(const string& s, const string& t);

Test cases

Input (s, t)	Output	Explanation
("egg", "add")	true	e→a, g→d
("foo", "bar")	false	o would map to both a and r
("paper", "title")	true	p→t, a→i, e→l, r→e
("badc", "baba")	false	b→b and d→b: two letters share a target
("a", "a")	true
("ab", "aa")	false	The reverse direction fails

(Confirm: s and t have the same length; characters are ASCII; a character may map to itself.)

How interviewers phrase it

• "Determine whether two strings are isomorphic" (LeetCode 205).
• Related: "word pattern" (LeetCode 290), where letters map to words.

Clarifying questions to ask

1. Are the strings the same length? (If not, false.)
2. Which character set: lowercase letters, ASCII, Unicode?
3. May a character map to itself?

Follow-ups to expect

1. "Why check both directions?"
2. "Is there a trick with one pass and no maps?"
3. "What's another characterisation?"
4. "Complexity?"

Related questions

• Check if two strings are anagram of each other Easy
• Sort Characters by Frequency Easy
• Roman to Integer Medium
• Sum of Beauty of All Substrings Medium
• Longest Repeating Character Replacement Hard
• Longest Substring With At Most K Distinct Characters Hard

All 474 dsa interview questions

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