The problem belongs to LeetCode, here is the link to the problem

Following code segment contains the description in the class comments, followed by fully working solution

/**
 * 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").
 */
public class CheckInclusion {
    public boolean checkInclusion(String s1, String s2) {
        if (s1.length() > s2.length()) return false;
        char[] s1Arr = s1.toCharArray();
        char[] s2Arr = s2.toCharArray();
        int[] buck1 = new int[256];
        int[] buck2 = new int[256];
        for (char c : s1Arr) {
            buck1[c]++;
        }
        for (int j = 0; j < s1Arr.length - 1; j++) {
            buck2[s2Arr[j]]++;
        }
        for (int i = 0; i < 1 + s2Arr.length - s1Arr.length; i++) {
            boolean match = true;
            buck2[s2Arr[i + s1Arr.length - 1]]++;
            for (char c : s1Arr) {
                if (buck1[c] != buck2[c]) {
                    match = false;
                    break;
                }
            }
            if (match) return true;
            buck2[s2Arr[i]]--;
        }
        return false;
    }
}