Longest Common Substring

string1 = 'thisisatest'
string2 = 'testing123testing'

results = [[None] * len(string2) for i in xrange(len(string1))]

def solve(string1, string2, i1, i2):
    if i1 == len(string1) or i2 == len(string2): return ''
    if results[i1][i2] is None:
        if string1[i1] == string2[i2]:
            results[i1][i2] = string1[i1] + solve(string1, string2, i1 + 1, i2 + 1)
        else:
            rright = solve(string1, string2, i1, i2 + 1)
            rleft = solve(string1, string2, i1 + 1, i2)
            results[i1][i2] = rleft if len(rleft) > len(rright) else rright
    return results[i1][i2]
print solve(string1, string2, 0, 0)