---
title: '567. Permutation in String'
description: Given two strings s1 and s2, return true if s2 contains a permutation of s1, or false otherwise
sidebar:
  label: 'Permutation in String'
  badge: 'Medium'
---

<Badge variant="accent">Sliding Window</Badge>

### Example 1:
- Input: `s1 = "ab", s2 = "eidbaooo"`
- Output: `true`
- Explanation: `s2` contains one permutation of `s1` ("ba").

### Example 2:
- Input: `s1 = "ab", s2 = "eidboaoo"`
- Output: `false`

### Constraints:

- `1 <= s1.length, s2.length <= 10^4`
- `s1` and `s2` consist of lowercase English letters.

## Solution

```py
from collections import Counter


class Solution:
    def checkInclusion(self, s1: str, s2: str) -> bool:
        k = len(s1)
        if k > len(s2):
            return False

        need = Counter(s1)
        window = Counter()

        for right, c in enumerate(s2):
            window[c] += 1
            if right >= k:
                left_char = s2[right - k]
                window[left_char] -= 1
                if window[left_char] == 0:
                    del window[left_char]
            if window == need:
                return True

        return False
```