Add longest common substring (#7488)

* added longest common substring

* added retrun type hint

* Update dynamic_programming/longest_common_substring.py

Co-authored-by: Caeden Perelli-Harris <caedenperelliharris@gmail.com>

* Update dynamic_programming/longest_common_substring.py

Co-authored-by: Caeden Perelli-Harris <caedenperelliharris@gmail.com>

* Update dynamic_programming/longest_common_substring.py

Co-authored-by: Caeden Perelli-Harris <caedenperelliharris@gmail.com>

* Update dynamic_programming/longest_common_substring.py

Co-authored-by: Caeden Perelli-Harris <caedenperelliharris@gmail.com>

* Update dynamic_programming/longest_common_substring.py

Co-authored-by: Caeden Perelli-Harris <caedenperelliharris@gmail.com>

* Update dynamic_programming/longest_common_substring.py

Co-authored-by: Caeden Perelli-Harris <caedenperelliharris@gmail.com>

* Update dynamic_programming/longest_common_substring.py

Co-authored-by: Caeden Perelli-Harris <caedenperelliharris@gmail.com>

* Update dynamic_programming/longest_common_substring.py

Co-authored-by: Caeden Perelli-Harris <caedenperelliharris@gmail.com>

* Update dynamic_programming/longest_common_substring.py

Co-authored-by: Caeden Perelli-Harris <caedenperelliharris@gmail.com>

* Update dynamic_programming/longest_common_substring.py

Co-authored-by: Caeden Perelli-Harris <caedenperelliharris@gmail.com>

* Update dynamic_programming/longest_common_substring.py

Co-authored-by: Caeden Perelli-Harris <caedenperelliharris@gmail.com>

* changed t1, t2 to text1, text2

* Update longest_common_substring.py

* Update dynamic_programming/longest_common_substring.py

Co-authored-by: Chris O <46587501+ChrisO345@users.noreply.github.com>

* Update dynamic_programming/longest_common_substring.py

Co-authored-by: Chris O <46587501+ChrisO345@users.noreply.github.com>

* [pre-commit.ci] auto fixes from pre-commit.com hooks

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* applied suggested changes

* Update dynamic_programming/longest_common_substring.py

Co-authored-by: Caeden Perelli-Harris <caedenperelliharris@gmail.com>

* removed space between line

* return longest common substring

* Update dynamic_programming/longest_common_substring.py

Co-authored-by: Christian Clauss <cclauss@me.com>

* [pre-commit.ci] auto fixes from pre-commit.com hooks

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Co-authored-by: Caeden Perelli-Harris <caedenperelliharris@gmail.com>
Co-authored-by: Chris O <46587501+ChrisO345@users.noreply.github.com>
Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>
Co-authored-by: Christian Clauss <cclauss@me.com>

dynamic_programming/longest_common_substring.py

@@ -0,0 +1,63 @@
+"""
+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()