Update README.md

Author: assassint2017

README.md

@@ -117,5 +117,5 @@
 100. []()
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 136. [Single Number](https://github.com/assassint2017/leetcode-algorithm/tree/master/Single%20Number)
-202. [Happy Number]()
+202. [Happy Number](https://github.com/assassint2017/leetcode-algorithm/tree/master/Happy%20Number)
 415. [Add Strings](https://github.com/assassint2017/leetcode-algorithm/tree/master/Add%20Strings)