567. Permutation in String

Given two strings s1 and s2, return true if s2 contains a permutation of s1, or false otherwise.

In other words, return true if one of s1's permutations is the substring of s2.

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 <= 104
• s1 and s2 consist of lowercase English letters.

Solution:

class Solution {
    public boolean checkInclusion(String s1, String s2) {
        /*
        s1 = "ab"
        s2 = "eidbaooo"


        */
        if (s1.length() > s2.length()){
            return false;
        }

        int[] arr1 = new int[26];
        int[] arr2 = new int[26];

        for (int i = 0; i < s1.length(); i++){
            arr1[s1.charAt(i) - 'a']++;
            arr2[s2.charAt(i) - 'a']++;
        }

        if (Arrays.equals(arr1, arr2)){
            return true;
        }

        int front = 0;
        int back = s1.length();

        while(back < s2.length()){
            arr2[s2.charAt(front) - 'a']--;
            arr2[s2.charAt(back) - 'a']++;

            if (Arrays.equals(arr1, arr2)){
                return true;
            }

            front++;
            back++;
        }

        return false;
    }
}
// TC: O(n)
// SC: O(n)