Description

Added the Longest Common Substring algorithm using dynamic programming.

Difference from existing code:

The repo already has longestcommonsubsequence.go (Longest Common Subsequence), but Longest Common Substring is a different problem - the substring must be contiguous in both strings, whereas subsequence allows non-contiguous matches.

Implementation:

• Time complexity: O(m * n)
• Space complexity: O(m * n)
• Uses a DP table where dp[i][j] represents the length of the longest common substring ending at str1[i-1] and str2[j-1]

Test Coverage:

12 test cases covering:

• Basic matching
• Full string match
• No match
• Empty strings
• Single character cases
• Substring at start/end
• Repeated characters
• Overlapping patterns

All tests pass: go test ./dynamic/ -run TestLongestCommonSubstring -v

Checklist

• Code follows project style guide (STYLE.md)
• Includes comprehensive tests
• Proper documentation with time/space complexity
• No external dependencies