Update README.md

Author: assassint2017

README.md

@@ -24,7 +24,7 @@
 15. [3Sum](https://github.com/assassint2017/leetcode-algorithm/tree/master/3Sum)
 16. [3Sum Closest](https://github.com/assassint2017/leetcode-algorithm/tree/master/3Sum%20Closest)
 17. [Letter Combinations of a Phone Number](https://github.com/assassint2017/leetcode-algorithm/tree/master/Letter%20Combinations%20of%20a%20Phone%20Number)
-18. [4Sum]()
+18. [4Sum](https://github.com/assassint2017/leetcode-algorithm/tree/master/4Sum)
 19. [Remove Nth Node From End of List](https://github.com/assassint2017/leetcode-algorithm/tree/master/Remove%20Nth%20Node%20From%20End%20of%20List)
 20. [Valid Parentheses](https://github.com/assassint2017/leetcode-algorithm/tree/master/Valid%20Parentheses)
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