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def lcs(xstr, ystr):
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"""
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>>> lcs('thisisatest', 'testing123testing')
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'tsitest'
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"""
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if not xstr or not ystr:
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return ""
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x, xs, y, ys = xstr[0], xstr[1:], ystr[0], ystr[1:]
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if x == y:
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return str(lcs(xs, ys)) + x
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else:
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return max(lcs(xstr, ys), lcs(xs, ystr), key=len)
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