Add: General, C, Graph categories.

C.md

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+## Memory Alignment & Struct Padding
+- http://www.catb.org/esr/structure-packing/

General.md

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+## Cache Memory
+- <www.geeksforgeeks.org/computer-science-fundamentals/cache-memory/>

Graph.md

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+## Directed Acyclic Graphs
+- https://ocw.mit.edu/courses/6-042j-mathematics-for-computer-science-spring-2015/mit6_042js15_session17.pdf
+- https://www.geeksforgeeks.org/dsa/introduction-to-directed-acyclic-graph/