Ticket twins

Given two strings a and b of equal length, containing only lowercase Latin letters, determine whether they are isomorphic. Each letter in a must consistently correspond to one letter in b at the same positions, and different letters in a must correspond to different letters in b. Return a boolean. Two empty strings are isomorphic.

Example:

ticket_twins("aab", "xxy")  →  True

aab and xxy: two a’s map to two x’s, b maps to y.

ticket_twins("aba", "xyy")  →  False

aba and xyy fail because the first string’s first and last letters are the same, but x and y are not.

Pattern: hashmap - map each letter to its partner and check consistency both ways

Need a hint?

Reveal one step at a time. Stop when you have enough to keep going.

1 · What to notice

A consistent mapping in one direction is not enough: two different source letters must not share one target letter. How can you check both restrictions?

2 · Walk through an example

For egg → add, record e → a and g → d; the second g must still map to d. For ab → cc, a → c and b → c look consistent individually, but both use c. A reverse map c → a reveals the conflict when b arrives.

3 · Pseudocode and self-check

Language-independent pseudocode. Indices start at 0; ← means assignment. Ranges with an upper bound exclude that bound.

forward ← {}
reverse ← {}
FOR i FROM 0 TO LENGTH(a) EXCLUSIVE:
    x ← a[i]
    y ← b[i]
    IF x IN forward AND forward[x] ≠ y:
        RETURN false
    IF y IN reverse AND reverse[y] ≠ x:
        RETURN false
    forward[x] ← y
    reverse[y] ← x
RETURN true

Check your reasoning

Two empty strings → true. ab/cc → false; aa/ab → false. Use both forward and reverse mappings. Time O(n), auxiliary space O(1) for the fixed lowercase alphabet.

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Ticket twins

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