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

Note:

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

Solution

public class Solution {
      public boolean checkInclusion(String s1, String s2) {
        if (s2.length() < s1.length()) {
            return false;
        }

        int[] f = new int[26];
        int[] g = new int[26];

        for (int i = 0; i < s1.length(); i++) {
            f[s1.charAt(i) - 'a'] += 1;
        }

        int s1Size = s1.length();
        for (int i = 0; i < s1Size; i++) {
            g[s2.charAt(i) - 'a'] += 1;
        }

        if (Arrays.equals(f, g)) {
            return true;
        }


        for (int i = 1; i + s1Size <= s2.length(); i++) {
            g[s2.charAt(i - 1) - 'a'] -= 1;
            g[s2.charAt(i + s1Size - 1) - 'a'] += 1;

            if (Arrays.equals(f, g)) {
                return true;
            }
        }

        return false;
    }
}