Update README.md

Author: assassint2017

README.md

@@ -4,7 +4,7 @@
 # leetcode-algorithm
 |total|easy|medium|hard|
 |:---:|:---:|:---:|:---:|
-|106|33|61|12|
+|108|33|62|13|
 ----
 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)
@@ -133,5 +133,6 @@
 119. [Pascal's Triangle II](https://github.com/assassint2017/leetcode-algorithm/tree/master/Pascal's%20Triangle%20II)
 120. [Triangle](https://github.com/assassint2017/leetcode-algorithm/tree/master/Triangle)
 136. [Single Number](https://github.com/assassint2017/leetcode-algorithm/tree/master/Single%20Number)
+144. [Binary Tree Preorder Traversal](https://github.com/assassint2017/leetcode-algorithm/tree/master/Binary%20Tree%20Preorder%20Traversal)
 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)