public class Solution {
    public string MinWindow(string s, string t) {
        if (s == null | t == null || s.Length == 0 || t.Length == 0 || t.Length > s.Length) return "";
        int n = s.Length; int m = t.Length;

        var need = new int[256]; int count = m;
        foreach(char c in t) need[c]++;

        int i = 0; int min_i = 0; long len = s.Length + 1;
        for (int j = 0; j < n; j++){
            if (need[s[j]]-- >= 1) count--; // Reducing need -affect counter only when need >= 1
            while(count == 0){ // All needs met - till need satsifed continue
                if (j-i+1 < len) { len = j-i+1; min_i = i; } // need valid
                if (need[s[i++]]++ == 0) count++; // Add to need, if need created - next break;
            }
        }
        return len == s.Length + 1 ? "" : s.Substring(min_i, (int)len);
    }
}