def trim(s):    flag=True    if s =="":        return s    for i in range(len(s)):        if s[i]!=' ':            s=s[i:]            flag=False            break    for i in range(-1,-len(s),-1):        if s[i]!=' ':            if i==-1:                s=s[:]            else:                s=s[:i+1]            flag=False            break    if flag ==True:        s=""    return s