Update README.md

Author: piyush6348

README.md

@@ -11,4 +11,6 @@ LeetCode
 |1|[Two Sum](https://leetcode.com/problems/two-sum/#/description)| [C++](https://github.com/piyush6348/leetcode/blob/master/Two%20Sum.cpp)|Easy|
 |101|[Symmetric Tree](https://leetcode.com/problems/symmetric-tree/#/description)| [C++](https://github.com/piyush6348/leetcode/blob/master/Symmetric%20Tree.cpp)|Easy|
 |404|[Sum of Left Leaves](https://leetcode.com/problems/sum-of-left-leaves/#/description)| [C++](https://github.com/piyush6348/leetcode/blob/master/Sum%20of%20Left%20Leaves.cpp)|Easy|
+|8|[String to integer atoi](https://leetcode.com/problems/string-to-integer-atoi/#/description)| [C++](https://github.com/piyush6348/leetcode/blob/master/String%20to%20Integer%20(atoi).cpp)|Medium|
+|451|[Sort Characters By Frequency](https://leetcode.com/problems/sort-characters-by-frequency/#/description)| [C++](https://github.com/piyush6348/leetcode/blob/master/Sort%20Characters%20By%20Frequency.cpp)|Medium|