LeetCode #395 Longest Substring

int longestSubstring(char * s, int k){
    int len = 0, n = strlen(s);
    if (k == 1) return n;
    int first[26], start[n], num[26];
    for (int i = 0; i < 26; i++) { 
        num[i] = 0;
        first[i] = -1;
    }
    for (int i = 0; i < n; i++) { start[i] = i; }
    
    for (int i = 0; i < n; i++) {
        int c = s[i] - 'a';
        if (first[c] == -1) { first[c] = i; }
        num[c]++;
        if (num[c] >= k) {
            int j = i - 1, left = first[c], min[26];
            for (int ic = 0; ic < 26; ic++) { min[ic] = 0; }
            min[c] = 1;
            while (j >= left && num[s[j] - 'a'] >= k) {
                min[s[j] - 'a']++;
                if (start[j] != j) { 
                    j = start[j]; 
                } else {
                    j--;
                } 
                if (j >= 0 && num[s[j] - 'a'] >= k && left > first[s[j] - 'a']) { left = first[s[j] - 'a']; }
            }
            if (j == left - 1) {
                int tlen = i - j;
                if (len < tlen) { len = tlen; }
            } else {
                int mink = n;
                for (int ic = 0; ic < 26; ic++) { if (min[ic] != 0 && min[ic] < mink) { mink = min[ic]; } }
                if (mink < n && mink >= k) {
                    int tlen = i - j;
                    if (len < tlen) { len = tlen; }
                }
            }
        }
    }
    return len;
}