Add longest common substring

dynamic_programming/longest_common_substring.py

@@ -0,0 +1,62 @@
+"""
+Longest Common Substring Problem Statement: Given two sequences, find the length
+of 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, the length is 2.
+"""
+
+
+def longest_common_substring(text1: str, text2: str) -> int:
+ """
+ Finds the length of longest common substring between two strings.
+
+
+ >>> longest_common_substring("", "")
+ 0
+ >>> longest_common_substring("a","")
+ 0
+ >>> longest_common_substring("", "a")
+ 0
+ >>> longest_common_substring("a", "a")
+ 1
+ >>> longest_common_substring("abcdef", "bcd")
+ 3
+ >>> longest_common_substring("abcdef", "xabded")
+ 2
+ >>> longest_common_substring("GeeksforGeeks", "GeeksQuiz")
+ 5
+ >>> longest_common_substring("abcdxyz", "xyzabcd")
+ 4
+ >>> longest_common_substring("zxabcdezy", "yzabcdezx")
+ 6
+ >>> longest_common_substring("OldSite:GeeksforGeeks.org", "NewSite:GeeksQuiz.com")
+ 10
+ >>> 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 = 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]
+ ans = max(ans, dp[i][j])
+
+ return ans
+
+
+if __name__ == "__main__":
+ import doctest
+
+ doctest.testmod()