class Solution {private:    int shd_cnt[256];    int cur_cnt[256];public:    string minWindow(string S, string T) {        int slen = S.length();        int tlen = T.length();                // array to store char stat info of string T        for (int i=0; i<256; i++) shd_cnt[i] = cur_cnt[i] = 0;        // collect char stat info of string T        for (int i=0; i<tlen; i++) shd_cnt[T[i]]++;                int shd_match_cnt = 0, cur_match_cnt = 0;        for (int i=0; i<256; i++) shd_match_cnt += shd_cnt[i] != 0;                int start = 0, end = 0;                int mlen = INT_MAX, ps = 0, pe = 0;                bool updated = true;                while (updated) {            updated = false;            while (cur_match_cnt == shd_match_cnt && start < slen) {                if (end - start < mlen) {                    mlen = end - start;                    ps = start, pe = end;                }                                char ch = S[start];                // whether current char is critical for matching                // if so it could not be omitted                if (cur_cnt[ch] - 1 < shd_cnt[ch]) break;                updated = true;                cur_cnt[ch]--;                start++;            }                        if (end < slen) {                updated = true;                char ch = S[end++];                                if (++cur_cnt[ch] == shd_cnt[ch]) { // using strict equal                    cur_match_cnt++;                }            }        }        return S.substr(ps, pe - ps);    }};