Find characters of s1 that are not present in s2 and append to result
for i in range ( 0 , len ( s1 ) ) : NEW_LINE INDENT if ( not s1 [ i ] in m ) : NEW_LINE INDENT res = res + s1 [ i ] NEW_LINE DEDENT else : NEW_LINE INDENT m [ s1 [ i ] ] = 2 NEW_LINE DEDENT DEDENT