Catch up with the trail pointer
Updated
Catching up is L += 1 while R stays. Repeat until L reaches the lead, or until the rule stops. The trail consumes cells the lead already passed.
What to focus on
After each catch-up step L is the old L+1, R is unchanged, and L ≤ R. The trail must not pass the lead.
Worked examples
one step
a = [4, 1, 9], L=0, R=2- start L=0, R=2
- L += 1
Result: L=1, R=2
caught up
a = [4, 1, 9], L=1, R=2- start L=1, R=2
- L += 1
Result: L=2, R=2
already together
a = [4, 1, 9], L=0, R=0- start L=0, R=0
- L += 1 would pass R
Result: L=0, R=0 · refuse
Try it
Write catchUp(a, L, R). a is an array of integers. L is the trail index. R is the lead index. Do L += 1. Print L and R after each step. Return nothing. On [4, 1, 9] from (0, 2) then (1, 2), and on [4, 1, 9, 1] from (0, 3).
a has length ≥ 3. Start with L < R.