0567-permutation-in-string.cpp

/*
    Given 2 strings, return true if s2 contains permutation of s1
    Ex. s1 = "ab", s2 = "eidbaooo" -> true, s2 contains "ba"

    Sliding window, expand + count down char, contract + count up char

    Time: O(n)
    Space: O(1)
*/

class Solution {
public:
    bool checkInclusion(string s1, string s2) {
        int m = s1.size();
        int n = s2.size();
        if (m > n) {
            return false;
        }
        
        vector<int> count(26);
        for (int i = 0; i < m; i++) {
            count[s1[i] - 'a']++;
            count[s2[i] - 'a']--;
        }
        if (isPermutation(count)) {
            return true;
        }
        
        for (int i = m; i < n; i++) {
            count[s2[i] - 'a']--;
            count[s2[i - m] - 'a']++;
            if (isPermutation(count)) {
                return true;
            }
        }
        
        return false;
    }
private:
    bool isPermutation(vector<int>& count) {
        for (int i = 0; i < 26; i++) {
            if (count[i] != 0) {
                return false;
            }
        }
        return true;
    }
};