Solution.java

class Solution {
    public int equalCountSubstrings(String s, int count) {
        int n = s.length();
        if (count > n) {
            return 0;
        }
        int[][] counter = new int[n + 1][26];
        int ans = 0;
        for (int i = 0; i < n; ++i) {
            int idx = s.charAt(i) - 'a';
            for (int j = 0; j < 26; ++j) {
                counter[i + 1][j] = counter[i][j];
            }
            counter[i + 1][idx] = counter[i][idx] + 1;
            int l = 0;
            for (int k = 0; k < 26; ++k) {
                l += count;
                int j = i - l + 1;
                if (j < 0) {
                    break;
                }
                ans += check(j, i, count, counter) ? 1 : 0;
            }
        }
        return ans;
    }

    private boolean check(int i, int j, int count, int[][] counter) {
        int[] c1 = counter[i];
        int[] c2 = counter[j + 1];
        for (int k = 0; k < 26; ++k) {
            if (c2[k] == 0 || c1[k] == c2[k]) {
                continue;
            }
            if (c2[k] - c1[k] != count) {
                return false;
            }
        }
        return true;
    }
}