Longest common subsequence

lcs.py

def lcs(str1, str2):
    return _lcs(str1, str2, 0, 0, "")

def _lcs(str1, str2, i, j, result):
    if i==len(str1) or j==len(str2):
        return result

    str1_char = str1[i]
    str2_char = str2[j]

    if str1_char == str2_char:
        return longest(str1, str2, i+1, j+1, result+str1_char)

    first_alternative = longest(str1, str2, i+1, j, result)
    second_alternative = longest(str1, str2, i, j+1, result)

    return len(first_alternative) > len(second_alternative) and first_alternative or second_alternative

print lcs("andy", "andy venet")
print lcs("argentina", "aarg")