Advance the lead pointer first
Updated
The lead is R. Same-direction scans move it first: R += 1. The trail L stays. You look ahead or grow a gap before the trail catches up.
What to focus on
After the move, R is the old R+1, L is unchanged, and L ≤ R. R stays < n.
Worked examples
from together
a = [4, 1, 9], L=0, R=0- start L=0, R=0
- R += 1
Result: L=0, R=1
again
a = [4, 1, 9], L=0, R=1- start L=0, R=1
- R += 1
Result: L=0, R=2
blocked
a = [4, 1, 9], L=0, R=2- start L=0, R=2
- R += 1 would be 3, out
Result: L=0, R=2 · refuse
Try it
Write advanceLead(a, L, R). a is an array of integers. L is the trail index. R is the lead index. Do R += 1. Print L and R after the 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.