Solution
Approach 1: Sort
Intuition and Algorithm
Create an array of the squares of each element, and sort them.
Complexity Analysis

Time Complexity: , where is the length of
A
. 
Space Complexity: .
Approach 2: Two Pointer
Intuition
Since the array A
is sorted, loosely speaking it has some negative elements with squares in decreasing order, then some nonnegative elements with squares in increasing order.
For example, with [3, 2, 1, 4, 5, 6]
, we have the negative part [3, 2, 1]
with squares [9, 4, 1]
, and the positive part [4, 5, 6]
with squares [16, 25, 36]
. Our strategy is to iterate over the negative part in reverse, and the positive part in the forward direction.
Algorithm
We can use two pointers to read the positive and negative parts of the array  one pointer j
in the positive direction, and another i
in the negative direction.
Now that we are reading two increasing arrays (the squares of the elements), we can merge these arrays together using a twopointer technique.
Complexity Analysis

Time Complexity: , where is the length of
A
. 
Space Complexity: .
Analysis written by: @awice.