Permutation in String

ID: 567

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.

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

Idea

Bitmap, not brute force

If s1 is longer than s2, cannot contain permutation

If character frequency in s1 equals to character frequency in a consecutive order in s2, is permutation, else false

Use int[26] to record character frequency of 26 characters

Code

public boolean checkInclusion(String s1, String s2) {
        if (s1.length() > s2.length())
            return false;
        int[] s1map = new int[26];
        for (int i = 0; i < s1.length(); i++)
            s1map[s1.charAt(i) - 'a']++;
        for (int i = 0; i <= s2.length() - s1.length(); i++) {
            int[] s2map = new int[26];
            for (int j = 0; j < s1.length(); j++) {
                s2map[s2.charAt(i + j) - 'a']++;
            }
            if (matches(s1map, s2map))
                return true;
        }
        return false;
    }
    
    public boolean matches(int[] s1map, int[] s2map) {
        for (int i = 0; i < 26; i++) {
            if (s1map[i] != s2map[i])
                return false;
        }
        return true;
    }