Check one of permutation of string1 in string2

class Solution {
public:
  bool checkInclusion(string s1, string s2) {
    int n = size(s1), m = size(s2);
    if (n > m) return false;
    vector<int> hash1(26, 0), hash2(26, 0);
    for (auto c : s1) hash1[c - 'a']++;
    int i = 0, j = 0;
    while (i < m) {
      hash2[s2[i] - 'a']++;
      if (i >= n) hash2[s2[j++] - 'a']--;
      if (hash1 == hash2) return true;
      i++;
    }
    return false;
  }
};