string minWindow(string s, string t) {

if (s.empty() || s.length() < t.length()) {

return "";

}

const int ASCII_MAX = 256;

vector<int> exp_cnt(ASCII_MAX, 0);

vector<int> cur_cnt(ASCII_MAX, 0);

int cnt = 0;

int start = 0;

int min_start = 0;

int min_width = numeric_limits<int>::max();

for (const auto& c : t) {

++exp_cnt[c];

}

for (int i = 0; i < s.length(); ++i) {

if (exp_cnt[s[i]] > 0) {

++cur_cnt[s[i]];

if (cur_cnt[s[i]] <= exp_cnt[s[i]]) { // Counting expected elements.

++cnt;

}

}

if (cnt == t.size()) { // If window meets the requirement.

while (exp_cnt[s[start]] == 0 || // Adjust left bound of window.

cur_cnt[s[start]] > exp_cnt[s[start]]) {

--cur_cnt[s[start]];

++start;

}

if (min_width > i - start + 1) { // Update minimum window.

min_width = i - start + 1;

min_start = start;

}

}

}

if (min_width == numeric_limits<int>::max()) {

return "";

}

return s.substr(min_start, min_width);

}