s = "BWABYXFYTTZDSGHYXEEZOOPXYZ"
t = "XYYZ"
from collections import Counter, defaultdict

def shortest_substring(st, pat):
    n = len(st)
    cnt = Counter(pat)
    keys = cnt.keys()
    dct = defaultdict(int)
    
    i, j = 0, 0
    min_len = float('inf')
    min_st = ""

    while j < n:
        dct[st[j]] += 1
        
        while all(dct[k] >= cnt[k] for k in keys):  # Valid window found
            if (j - i + 1) < min_len:
                min_len = j - i + 1
                min_st = st[i:j+1]
            
            dct[st[i]] -= 1
            if dct[st[i]] == 0:
                del dct[st[i]]  # Remove zero-count keys to keep dictionary clean
            i += 1
        
        j += 1
    
    return min_st

r = shortest_substring(s, t)
print(r)