added the solution for longest common substring in java

[Sahilphide]

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Description 📜

• added the dp solution of question Longest Common Substring in java

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Type of change 📝

• [x] Code
• [ ] Documentation (Content Creation in the form of codes or tutorials)
• [ ] Notes (Handwritten notes, cheatsheets, presentations, mind-maps)
• [ ] Web Dev Projects
• [ ] Others (If you choose other, Please mention changes below)

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Programming Language

• [ ] C#
• [ ] C++
• [ ] Python
• [x] Java
• [ ] Javascript
• [ ] Others

Domain of Contribution 📊

• [ ] Introduction
• [x] Data Structures
• [x] Algorithms
• [x] Competitive Programming
• [ ] Mathematics
• [ ] Bit Magic
• [ ] Recursion
• [ ] Arrays
• [ ] Searching
• [ ] Sorting
• [ ] Matrix
• [ ] Hashing
• [ ] String
• [ ] Linked List
• [ ] Stacks
• [ ] Queue
• [ ] Deque
• [ ] Tree
• [ ] Binary Search Tree
• [ ] Heap
• [ ] Graph
• [ ] Greedy
• [ ] Backtracking
• [x] Dynamic Programming
• [ ] Trie
• [ ] Segment Tree
• [ ] Disjoint Set
• [ ] Others

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Checklist ✅

• [x] I follow Contributing Guidelines & Code of conduct of this project.
• [x] I have performed a self-review of my own code or work.
• [x] I have commented my code, particularly in hard-to-understand areas.
• [x] My changes generates no new warnings.

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Screenshots / Gif (Optional) 📸

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