Added info to btree.md
added hash table notes
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Formatting fix
More formatting
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Update single_linked_list.md

Shaai and Shayna's explanation of singly-linked lists
Merge pull request #5 from raielin/master

Binary Tree
Merge pull request #4 from ShaynaCummings/patch-1

Update single_linked_list.md
Merge pull request #3 from CollinBlinder/patch-1

Update doubly_linked_list.md
Merge pull request #2 from AReddy14/master

Added Info to btree.md
Merge pull request #1 from rpazyaquian/master

added hash table notes