def lenLongestSubstring( stringX ):
    maxi = 0
    trackList = [] # List to store the chars that have been visited  
    for char in stringX :
        if ( not ( char in trackList ) ):
            trackList.append(char) 
        else: # found a repeating char
            listLen = len(trackList)
            maxi = max( maxi, listLen )
            trackList[:] = [] # clears the list
    return max( maxi, listLen )
 
 
def main():
    assert lenLongestSubstring( "BBBBB" ) == 1
    assert lenLongestSubstring( "ABDEFGABEF" ) == 6
    assert lenLongestSubstring( "abcabcbb" ) == 3 
    print "\nAll asserts PASSED!!, Yaaaaaay!!\n"
 
if __name__ == "__main__":
    main()

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