https://leetcode.com/problems/longest-substring-with-at-least-k-repeating-characters/public class Solution {    public int longestSubstring(String s, int k) {        // Find count of each char        HashMap mp = new HashMap();        Object intObj;        for (int i=0; i<s.length(); i++) {            char ch = s.charAt(i);            intObj = mp.remove(ch);            int st = 0;            if (intObj != null) {                st = (int) intObj;            }            st++;            mp.put(ch, st);        }                // prepre iterate secondly        int ret = 0;        int last = -1;        HashMap newMp = new HashMap();                // iterate secondly        for (int i=0; i<s.length(); i++) {            char ch = s.charAt(i);            int num = (int)mp.get(ch);                        // pass if num fits            if (num >= k) {                intObj = newMp.get(ch);                int newNum = 0;                if (intObj != null) {                    newNum = (int) intObj;                }                newNum++;                newMp.put(ch, newNum);                continue;            }                        // handle if meets nofit char            Set filter = new HashSet();            Iterator iter = newMp.entrySet().iterator();            Map.Entry entry;                        // check newMp and prepare filter            while (iter.hasNext()) {                entry = (Map.Entry) iter.next();                char cch = (char)entry.getKey();                int cnt = (int)entry.getValue();                                if (cnt < k) {                    filter.add(cch);                }                                int allCnt = (int)mp.remove(cch);                allCnt -= cnt;                mp.put(cch, allCnt);            }                        // Prune            if (filter.size() == newMp.size()) {                last = i;                newMp.clear();                continue;            }                        // use filter to check each segment            HashMap fMp = new HashMap();            int newLast = last;            for (int j=last+1; j<=i; j++) {                char fch = ‘ ‘;                if (j < i) {                    fch = s.charAt(j);                }                                // need to check segment                if (j == i || filter.contains(fch)) {                    iter = fMp.entrySet().iterator();                    boolean fits = true;                                        // check map of each segment                    while (iter.hasNext()) {                        entry = (Map.Entry) iter.next();                        char ffch = (char)entry.getKey();                        int fcnt = (int)entry.getValue();                                                                if (fcnt < k) {                            fits = false;                        }                                                // Prepare Prune by update newMp                                              int newCnt = (int)newMp.remove(ffch);                        newCnt -= fcnt;                        newMp.put(ffch, newCnt);                                                if (newCnt < k) {                            filter.add(ffch);                        }                    }                                        // update final ret                    if (fits) {                        if (j-newLast-1 > ret) {                            ret = j-newLast-1;                        }                    }                    newLast = j;                    fMp.clear();                                        // Check Prune                    if (filter.size() == newMp.size()) {                        break;                    }                            }                // no need to check segment, pass                else {                    intObj = fMp.get(fch);                    int fNum = 0;                    if (intObj != null) {                        fNum = (int) intObj;                    }                    fNum++;                    fMp.put(fch, fNum);                }            }            newMp.clear();            last = i;                    }        if (s.length()-last-1 > ret) {            ret = s.length()-last-1;        }        return ret;    }}test case:"zzzzzzzzzzaaaaaaaaabbbbbbbbhbhbhbhbhbhbhicbcbcibcbccccccccccbbbbbbbbaaaaaaaaafffaahhhhhiaahiiiiiiiiifeeeeeeeeee"10expected return: 21