Improve longest_common_substring.py (#12705)

* Update longest_common_substring.py

- Combined the ans_index and ans_length into a single tuple to track the best match (position + length) more cleanly.

- Early exit for empty strings.

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

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* Update longest_common_substring.py

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

for more information, see https://pre-commit.ci

---------

Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>
Co-authored-by: Maxim Smolskiy <[email protected]>

Author: 3 people

dynamic_programming/longest_common_substring.py

@@ -43,22 +43,25 @@ def longest_common_substring(text1: str, text2: str) -> str:
     if not (isinstance(text1, str) and isinstance(text2, str)):
         raise ValueError("longest_common_substring() takes two strings for inputs")
 
+    if not text1 or not text2:
+        return ""
+
     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
+    end_pos = 0
+    max_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]
+                if dp[i][j] > max_length:
+                    end_pos = i
+                    max_length = dp[i][j]
 
-    return text1[ans_index - ans_length : ans_index]
+    return text1[end_pos - max_length : end_pos]
 
 
 if __name__ == "__main__":