Include the new rightmost cell
Updated
After R += 1, the cell a[R] now sits in the window. It was outside. Now it is inside. Name that cell.
What to focus on
Include means the new R is part of a[L..R]. You do not include a[R+1]. You do not skip a[R]. L is unchanged.
Worked examples
include first
a = [4, 1, 9], L=0, R from -1 to 0- new rightmost index is 0
- cell 4 enters
Result: window holds 4
include next
a = [4, 1, 9], L=0, R from 0 to 1- new rightmost index is 1
- cell 1 enters
- 4 stays
Result: window holds 4, 1
include on a string
s = "abca", L=0, R from 1 to 2- new rightmost index is 2
- char c enters
Result: window holds a, b, c
Try it
Write includeRight(a, L, R). a is an array of integers, or s is a string. L is the left index. R is the new right index after expand. The window now includes a[R]. Print the cell that entered. Return nothing. L is unchanged. On [4, 1, 9] expanding R=-1→0 then 0→1; "abca" expanding R=1→2.
a (or s) is nonempty. The expand R += 1 stays in range.