diff --git a/arrays_and_strings/README.md b/arrays_and_strings/README.md
index b482df9..f1136de 100644
--- a/arrays_and_strings/README.md
+++ b/arrays_and_strings/README.md
@@ -1,5 +1,37 @@
 ## arrays and strings
 
+<br>
+
+### comparing strings
+
+<br>
+
+* "==" can be used to compare two strings only if the language support operator overloading (like C++).
+
+<br>
+
+----
+
+### two-pointer technique
+
+<br>
+
+* a typical scenario is when you want to iterate the array from two ends to the middle. 
+* another secnario is when you need one slow-runner and one fast-runner at the same time (so that you can determine the movement strategy for both pointers).
+* in any case, this technique is usually used when the array is sorted.
+
+<br>
+
+---
+
+### two-dimensional arrays
+
+<br>
+
+* in some languages (like C++), 2d arrays are represented as 1d, so an array of `m * n` elements represents `array[i][j]` as `array[i * n + j]`.
+* dynamic 2d arrays a nested dynamic array.
+
+
 <br>
 
 ---