Isomorphic Strings
An easy Strings problem included in Striver A2Z. Below: the roles whose interviews prioritise this topic, and how to practise it.
- Topic
- Strings
- Sheets
- 1
- Core for
- 13 roles
- Platform
- LeetCode
The problem
Determine whether two strings are isomorphic, meaning each character in the first string can be replaced to get the second string while preserving the order of characters.
Example 1
- Input
- s="egg", t="add"
- Output
- true
Example 2
- Input
- s="foo", t="bar"
- Output
- false
Example 3
- Input
- s="paper", t="title"
- Output
- true
Constraints
- 1 <= s.length <= 5*10^4
- s and t have the same length
How to think about it
Updated 2026-09-09Isomorphism requires a strict one-to-one bijection between characters in both directions. Recording a mapping from s to t is only half the check; multiple distinct characters in s must not collapse into the same character in t. Tracking the last seen position of each character in both strings exposes any discrepancy immediately.
Approaches, worst first
Two hash maps bijection
time O(n) · space O(1)
Maintain mapST to store s[i] -> t[i] and mapTS to store t[i] -> s[i]. At each index i, if either mapping exists and differs from the current character pair, return false; otherwise register both assignments.
Last-seen index arraysWrite this one
time O(n) · space O(1)
Maintain two integer arrays of size 256 initialized to zero, recording the 1-based index where each character was last encountered. At each step, if lastSeenS[s[i]] != lastSeenT[t[i]], the structural alignment is broken. Otherwise, update both to i + 1.
Where people lose marks · 3
- Only mapping s -> t: for 'badc' and 'baba', 'd' and 'c' both map to 'a', violating the bijection if reverse mapping t -> s is omitted.
- Characters span ASCII: ensure indexing supports values up to 256 rather than assuming only lowercase letters.
- Using 0 as both the default unvisited marker and valid index 0; use 1-based indexing (i + 1) to distinguish unvisited characters from characters seen at position 0.
The theory behind it
Strings — the ground this problem stands on. All Strings problems
What Strings is
A string is an ordered necklace of text characters, like letters printed along a ribbon of paper. Each character sits at an exact numeric slot, holding a glyph such as a letter, punctuation mark, or digit. In many programming languages, ribbons cannot be edited after creation, meaning changing a single character requires pressing an entirely new ribbon from scratch.
When to reach for it
Reach for string techniques when inputs consist of words, DNA sequences, serialized data formats, or sentences. Clues include questions testing palindromes, anagram matches, substring patterns, parenthesis balancing, or character frequency counts. Whenever an algorithm asks to transform capitalization, parse structured tokens, or compute edits between two phrases, string representations are the core subject.
How the pattern works
Think of characters as small integer codes ranging across standard character sets. Frequency tables with fixed sizes often replace heavy hash maps when tallying occurrences. For search tasks, maintain rolling state using character indices or sliding borders. When building output text through repeated appends, accumulate pieces inside a mutable list or string builder rather than concatenating strings directly, avoiding quadratic copy overhead.
What each operation costs
| Operation | Time |
|---|---|
| read character by index | O(1) |
| concatenate two strings of total length n | O(n) |
| compare two strings of length n | O(n) |
What usually goes wrong with Strings
- Concatenating strings inside a loop using the plus operator, which silently creates full copies on each iteration and turns linear routines into quadratic slowdowns.
- Assuming all characters fall strictly within lowercase English letters without validating spaces, uppercase variants, punctuation marks, or multi-byte unicode symbols.
- Confusing substring length with end index when slicing, causing unexpected off-by-one truncations in languages that take length versus exclusive end position.
Which roles need this problem
Strings is a core topic for these 13 roles — if you're targeting one of them, this problem is early in your path, not optional.
Secondary for 7 more roles, including Data Engineer, Data Analyst, Embedded / Firmware Engineer.
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