Repeated String Match
A medium Strings problem included in Love Babbar 450. 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
Find the minimum number of times string A must be repeated so that string B is a substring of the repeated string.
Example 1
- Input
- A="abcd", B="cdabcdab"
- Output
- 3
Example 2
- Input
- A="a", B="aa"
- Output
- 2
Example 3
- Input
- A="abc", B="wxyz"
- Output
- -1
Constraints
- 1 <= A.length, B.length <= 10^4
How to think about it
Updated 2026-09-09For B to fit inside a repeated string of A, the total length of the repetition must be at least as long as B. If repeating A up to that minimum length still leaves B straddling an outer boundary, appending A exactly one more time covers all possible offset alignments. If B is still not found after that extra copy, repeating further will never help.
Approaches, worst first
Character-by-character modular offset scanning
time O(n * m) · space O(1)
Test each possible start index i in A where B could match, comparing B[j] with A[(i + j) % A.length]. If a mismatch occurs, restart matching from the next starting index in A. Repeatedly verifying overlapping character sequences from scratch without prebuilt pattern search routines runs into worst-case quadratic comparisons.
Bounded string expansion with substring searchWrite this one
time O(n + m) · space O(n + m)
Repeat A until its accumulated length is >= B.length. Check if B is a substring; if so, return the repetition count. If not, append A once more and check again. If still absent, B cannot possibly appear as a substring of any repetition of A, so return -1.
Where people lose marks · 3
- Stopping right at ceil(B.length / A.length) without checking one additional copy: B may begin near the end of the first copy of A and terminate inside an extra copy at the tail.
- Characters in B that do not exist anywhere in A: checking character sets beforehand can save needless string concatenation.
- Excessive repetition in an unbounded while-loop: repeating beyond count + 1 wastes memory and risks time limit exceeded when B is impossible.
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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