如何找到字符串中的最小子串

def find_min_substring(s, target):
    target_count = {}
    for char in target:
        target_count[char] = target_count.get(char, 0) + 1
    
    left = 0
    min_length = float('inf')
    min_start = 0
    count = len(target)
    
    for right in range(len(s)):
        if s[right] in target_count:
            target_count[s[right]] -= 1
            if target_count[s[right]] >= 0:
                count -= 1
        
        while count == 0:
            if right - left + 1 < min_length:
                min_length = right - left + 1
                min_start = left
            
            if s[left] in target_count:
                target_count[s[left]] += 1
                if target_count[s[left]] > 0:
                    count += 1
            left += 1
    
    if min_length == float('inf'):
        return ""
    return s[min_start:min_start+min_length]