LinkedList(), ~LinkedList(), find(), and set()

Implement these methods in the lab, but had no idea if they would work.
Need to be tested latter.
Debugging find method

find node0 should give the first node instead of dummy node
Implement add() and debugging the previous code
Something must be wrong in my add()
add() and remove() work fine

Haven't got a chance to implement splice().
Did a part of the exercises

Not sure if they were right or not.
Clarification on spice()
Merge pull request #54 from YangLi1/master

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