Editorial: Permutation in String

by admin · Oct. 18, 2026

1 Intuition

Two strings are permutations of each other iff their letter counts match.

2 Approach

Slide a fixed-size window of length |s1| over s2 and compare letter counts as it moves.

3 Complexity Analysis

Time O(26 · |s2|), Space O(1).

4 Solution Code

class Solution:
    def checkInclusion(self, s1: str, s2: str) -> bool:
        from collections import Counter
        n = len(s1)
        need, window = Counter(s1), Counter(s2[:n])
        if window == need:
            return True
        for i in range(n, len(s2)):
            window[s2[i]] += 1
            window[s2[i - n]] -= 1
            if window == need:
                return True
        return False

5 Code Walkthrough

The reference solution below implements the approach step by step.