Grow the gap between pointers
Updated
Growing the gap advances only the lead: R += 1. Distance R - L increases by 1. The trail stays put.
What to focus on
After the move, R - L is one larger, L is unchanged, and R < n. Growing from a meeting cell is how a same-direction scan starts.
Worked examples
from zero
a = [4, 1, 9], L=0, R=0- start L=0, R=0 · gap 0
- R += 1
Result: L=0, R=1 · gap 1
again
a = [4, 1, 9], L=0, R=1- start L=0, R=1 · gap 1
- R += 1
Result: L=0, R=2 · gap 2
one
a = [7], L=0, R=0- start L=0, R=0
- R += 1 would be 1, out
Result: L=0, R=0 · gap 0
Try it
Write growGap(a, L, R). a is an array of integers. L is the trail index. R is the lead index. Gap is R - L. Do R += 1. Print gap after each move. Return nothing. On [4, 1, 9] from (0, 0) then (0, 1), and on [4, 1, 9, 1] from (0, 2).
a has length ≥ 3. R starts strictly before the last index.