Example
Input
s = 'oidbcaf', pattern = 'abc'
Output
True
The substring 'bca' (positions 3..5) is a permutation of 'abc'.
Full lesson preview
Check if a string contains a permutation of a pattern
Determine if any permutation of a given pattern exists as a substring in the input string using a frequency map and sliding window.
Python practice15 minStrings – Sliding Window & FrequencyIntermediateLast updated March 25, 2026
Problem statement
Given a string s and a pattern p, return True if s contains any permutation of p (i.e., any substring of s of length len(p) that is an anagram of p). Otherwise return False. Use frequency counts and a sliding window of size len(p) to check each candidate substring in O(n) time.
Task
Implement a window of fixed size (pattern length) and compare character frequencies efficiently to detect any permutation match.
Examples
Input format
A function call contains_permutation(s, pattern) where s and pattern are strings.
Output format
A boolean (True/False) indicating whether any permutation of pattern exists in s.
Constraints
0 <= len(s), len(pattern) <= 10^5. Aim for O(n) time and O(min(n, alphabet)) extra space. If pattern is longer than s return False.
Samples
Sample 1
Input
s = 'aaacb', pattern = 'abc'
Output
True
Substring 'acb' is a permutation of 'abc'.