Update README.md

Author: assassint2017

README.md

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 |total|easy|medium|hard|
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 |83|23|50|10|
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 1. [Two Sum](https://github.com/assassint2017/leetcode-algorithm/tree/master/Two%20Sum)
 2. [Add Two Numbers](https://github.com/assassint2017/leetcode-algorithm/tree/master/Add%20Two%20Numbers)
 3. [Longest Substring Without Repeating Characters](https://github.com/assassint2017/leetcode-algorithm/tree/master/Longest%20Substring%20Without%20Repeating%20Characters)