Array/sliding window update

ds/linear/array/README.md

@@ -1,12 +1,12 @@
 # Array
 
-- [Overview](#overview)
-- [Applications](#applications)
-- [Algorithms \& Techniques](#algorithms--techniques)
-  - [Two-Pointer](#two-pointer)
-  - [Circular Array](#circular-array)
-- [Design \& Implementation](#design--implementation)
-  - [Java](#java)
+<!-- TOC -->
+* [Array](#array)
+  * [Overview](#overview)
+  * [Applications](#applications)
+  * [Algorithms & Techniques](#algorithms--techniques)
+    * [Two-Pointer traversal](#two-pointer-traversal)
+<!-- TOC -->
 
 ## Overview
 
@@ -30,3 +30,71 @@ This makes it easier to calculate the position of each element by simply adding
 ![two pointer](2023-03-28-23-34-13.png)
 
 > Two pointers help us cover a sub-array within the main array.
+
+The strength of this technique is that we will never have more than $𝑂(𝑛)$ 
+iterations for the while loop because the pointers start $𝑛$ away from each other and move at least
+one step closer in every iteration. Therefore, if we can keep the work inside each iteration at $𝑂(1)$,
+we'll have linear runtime.
+
+### Sliding window
+
+#### The algorithm
+
+In sliding window, we maintain two variables `left` and `right`, which at any given time
+represent the current subarray under consideration. For expanding the size of the window,
+we increase right. For shrinking the window, we increase left.
+
+```
+function fn(arr):
+    left = 0
+    for (int right = 0; right < arr.length; right++):
+        Do some logic to "add" element at arr[right] to window
+
+        while WINDOW_IS_INVALID:
+            Do some logic to "remove" element at arr[left] from window
+            left++
+
+        Do some logic to update the answer
+```
+
+When the window must have a fixed size `k`:
+
+```
+function fn(arr, k):
+    curr = some data to track the window
+
+    // build the first window
+    for (int i = 0; i < k; i++)
+        Do something with curr or other variables to build first window
+
+    ans = answer variable, probably equal to curr here depending on the problem
+    for (int i = k; i < arr.length; i++)
+        Add arr[i] to window
+        Remove arr[i - k] from window
+        Update ans
+
+    return ans
+```
+
+#### Problems
+
+There is a very common group of problems involving subarrays that
+can be solved efficiently with sliding window.
+
+First, the problem will either explicitly or implicitly define criteria that make a subarray "valid".
+There are 2 components regarding what makes a subarray valid:
+
+1. __A constraint metric__: Some attribute of the array such as the number of unique elements, their frequency, etc.
+2. __A numeric restriction__ on the constraint metric.
+
+Second, the problem will ask you to find subarrays in some way:
+
+1. The most common task is to find __the best__ valid subarray based on some criteria.
+2. Find the number of valid subarrays.
+
+Some examples of common problems are:
+
+- Find the longest subarray with at most K distinct elements.
+- Find the longest subarray with the sum less than or equal to `k`.
+- Find the longest substring that has at most one `0`.
+- Find the number of subarrays that have a product less that `k`.