Minimum_Window_Substring.py

class Solution:
    def minWindow(self, s: str, t: str) -> str:
        if t == "": return ""
        
        countT, window = {}, {}
        
        for c in t:
            countT[c] = 1 + countT.get(c, 0)
        
        have, need = 0, len(countT)
        res , reslen = [-1, -1], float('inf')
        l = 0  
        for r in range(len(s)):
            c = s[r]
            window[c] = 1 + window.get(c, 0)
            
            if c in countT and window[c] == countT[c]:
                have += 1
            
            while have == need:
                # update the result
                if (r - l + 1) < reslen:
                    res = [l, r]
                    reslen = r - l + 1
                # shrink the windows size
                window[s[l]] -= 1
                if s[l] in countT and window[s[l]] < countT[s[l]]:
                    have -= 1
                l += 1
                
        l, r = res
        return s[l : r + 1] if reslen != float('inf') else ""