#### Longest Common Substring

R
```"""
Longest Common Substring Problem Statement: Given two sequences, find the
longest common substring present in both of them. A substring is
necessarily continuous.
Example: "abcdef" and "xabded" have two longest common substrings, "ab" or "de".
Therefore, algorithm should return any one of them.
"""

def longest_common_substring(text1: str, text2: str) -> str:
"""
Finds the longest common substring between two strings.
>>> longest_common_substring("", "")
''
>>> longest_common_substring("a","")
''
>>> longest_common_substring("", "a")
''
>>> longest_common_substring("a", "a")
'a'
>>> longest_common_substring("abcdef", "bcd")
'bcd'
>>> longest_common_substring("abcdef", "xabded")
'ab'
>>> longest_common_substring("GeeksforGeeks", "GeeksQuiz")
'Geeks'
>>> longest_common_substring("abcdxyz", "xyzabcd")
'abcd'
>>> longest_common_substring("zxabcdezy", "yzabcdezx")
'abcdez'
>>> longest_common_substring("OldSite:GeeksforGeeks.org", "NewSite:GeeksQuiz.com")
'Site:Geeks'
>>> longest_common_substring(1, 1)
Traceback (most recent call last):
...
ValueError: longest_common_substring() takes two strings for inputs
"""

if not (isinstance(text1, str) and isinstance(text2, str)):
raise ValueError("longest_common_substring() takes two strings for inputs")

text1_length = len(text1)
text2_length = len(text2)

dp = [[0] * (text2_length + 1) for _ in range(text1_length + 1)]
ans_index = 0
ans_length = 0

for i in range(1, text1_length + 1):
for j in range(1, text2_length + 1):
if text1[i - 1] == text2[j - 1]:
dp[i][j] = 1 + dp[i - 1][j - 1]
if dp[i][j] > ans_length:
ans_index = i
ans_length = dp[i][j]

return text1[ans_index - ans_length : ans_index]

if __name__ == "__main__":
import doctest

doctest.testmod()
```