> For the complete documentation index, see [llms.txt](https://mayanktyagi3111.gitbook.io/interview-prep/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://mayanktyagi3111.gitbook.io/interview-prep/strings/permutation-in-string.md).

# Permutation in String

Given two strings **s1** and **s2**, write a function to return true if **s2** contains the permutation of **s1**. In other words, one of the first string's permutations is the **substring** of the second string.

**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:**

* The input strings only contain lower case letters.
* The length of both given strings is in range \[1, 10,000].

```java
class Solution {
    public boolean checkInclusion(String s1, String s2) {
        int count = s1.length();
        int[] freq = new int[26];
        boolean[] isPresent = new boolean[26];
        for (char x : s1.toCharArray()) {
            isPresent[x - 'a'] = true;
            freq[x - 'a']++;
        }
        int start = 0, end = 0;
        while (end < s2.length()) {
            char c = s2.charAt(end);
            if (freq[c - 'a'] > 0) {
                freq[c - 'a']--;
                count--;
                if (count == 0)
                    return true;
            } else {
                while (start <= end) {
                    if (s2.charAt(start) == c) {
                        start++;
                        break;
                    }
                    freq[s2.charAt(start) - 'a']++;
                    start++;
                    count++;
                }
            }
            end++;
        }
        return false;
    }
}
```