Implement strStr()
An easy Strings problem included in Love Babbar 450, Striver A2Z. Below: the roles whose interviews prioritise this topic, and how to practise it.
- Topic
- Strings
- Sheets
- 2
- Core for
- 13 roles
- Platform
- LeetCode
The problem
Find the index of the first occurrence of the needle string within the haystack string. Return -1 if needle is not part of haystack.
Example 1
- Input
- haystack="sadbutsad", needle="sad"
- Output
- 0
Example 2
- Input
- haystack="leetcode", needle="leeto"
- Output
- -1
Example 3
- Input
- haystack="hello", needle="ll"
- Output
- 2
Constraints
- 1 <= haystack.length, needle.length <= 10^4
- strings consist of lowercase English letters
How to think about it
Updated 2026-09-09When characters match up to index j and then fail, throwing away all matched knowledge and resetting the haystack pointer is what makes naive search quadratic. A prefix table on the needle tells you the longest proper prefix that is also a suffix, letting you shift the pattern pointer back to a known viable prefix without ever backing up in the text.
Approaches, worst first
Naive sliding comparison
time O(n * m) · space O(1)
Slide needle across haystack one position at a time and compare character by character. On repetitive inputs like needle 'aab' against haystack 'aaaaab', it constantly rewinds needle and repeats character comparisons.
Knuth-Morris-Pratt pattern matchingWrite this one
time O(n + m) · space O(m)
Precompute the longest prefix-suffix (LPS) table for needle in linear time. During haystack traversal, advance needle pointer j on matches. On mismatch, roll j back to lps[j - 1] while haystack pointer i advances continuously without backtracking.
Where people lose marks · 3
- Needle longer than haystack: the loop must exit immediately returning -1 without accessing out-of-bounds indices.
- Needle pointer j reaching needle length indicates a match at index i - needle.length.
- Rolling back j on mismatch: when j = 0, advance i directly instead of indexing lps[-1].
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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