Not claiming to be the fastest, but here are two Python solutions suggested by Dave Beazley [0], plus one I extended using compiled regular expressions:

    # string replacement
    s = ' hello  world \n'
    s.replace(' ', '')
    # 'helloworld'

    # regular expression
    import re
    re.sub('\s+', '', s)
    # 'helloworld'

    # compiled regular expression
    pat = re.compile('\s+')
    pat.sub('', s)
    # 'helloworld'

[0] https://www.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&c...