1542. Find Longest Awesome Substring

class Solution:
    def longestAwesome(self, s: str) -> int:
        """   tc O(KN)  sc O(2^K),  K: unique characters in string, here K = 10
        
        Note,  since palindrom allow up to one number with odd count  , check all masks different from current one by one bit 
        => if two masks are different by one bit, there is one odd cnt in the substring ????
        
        """
        
        
        res,mask,n = 0,0,len(s)
        seen = [n]*1024 
        
        seen[0] =-1 # position for 0, mask is -1 
        for i in range(n):
            #(1) update mask with current char  
            mask ^= 1 << int(s[i])
            
            #(3.1) check cases allow one number with odd count 
            # by using any number as middle char in palindrom, and check if its seen before  
            for j in range(10):
                res = max(res,i-seen[mask^(1<<j)])
            #(3.2) or non odd middle character 
            res = max(res,i-seen[mask])
        
            #(2) keep mask's index if it is the first time to show up  
            seen[mask] = min(seen[mask],i)
        return res