Minimum Window Substring

👶 The 5-Year-Old Summary

Find smallest window containing all characters of "t"! "ADOBECODEBANC" with "ABC" → "BANC"!

🎮 Interactive Visualizer

s = "ADOBECODEBANC", t = "ABC"

🧠 The Brain Hack

Expand right, shrink left! Use hash map to count required chars. When all chars found, shrink from left while still valid. Track smallest window!

💻 The Clean Solution

def min_window(s, t):
    if not t or not s: return ""
    
    # Count required chars
    need = Counter(t)
    window = {}
    
    have, needlen = 0, len(need)
    left = 0
    result = float('inf'), None
    
    for right in range(len(s)):
        c = s[right]
        window[c] = window.get(c, 0) + 1
        
        if c in need and window[c] == need[c]:
            have += 1
        
        while have == needlen:
            # Update result
            if right - left + 1 < result[0]:
                result = (right - left + 1, s[left:right+1])
            
            # Shrink window from left
           [s[left]] -= 1
            if s[left] in need and window[s[left]] < need[s[left]]:
                have -= 1
            left += 1
    
    return result[1] if result[1] else ""

# "ADOBECODEBANC", "ABC" → "BANC"

Time: O(n)

Space: O(alphabet)